PHYSICAL     SCIENCE 


Frontispiece 


THE    RECENT 
DEVELOPMENT     OF 
PHYSICAL    SCIENCE 


BY  WILLIAM  CECIL  DAMPIER  WHETHAM 

M.A.,  F.R.S. 

FELLOW  OF  TRINITY  COLLEGE,  CAMBRIDGE 


PHILADELPHIA 

P.   BLAKISTON'S   SON    b5   CO. 

1012  WALNUT   STREET 

1904 


PRINTED  IN  GREAT  BRITAIN 


PREFACE 

IN  recent  years  we  have  witnessed  a  great  develop- 
ment of  physical  science.  The  different  sections 
into  which  physical  knowledge  is,  for  the  sake  of 
convenience,  divided,  have  grown  each  within  its 
own  domain  ;  and,  moreover,  have  shown  increas- 
ing signs  of  extending  beyond  the  boundaries 
arbitrarily  traced  between  them.  The  methods  of 
physics,  in  the  restricted  sense  of  that  word,  are 
being  more  and  more  applied  to  chemical  and 
biological  problems,  while  many  questions  in 
physics  can  only  be  investigated  by  those  with 
mathematical  or  chemical  training. 

Thus  it  happens  that  an  acquaintance  with  the 
knowledge  newly  acquired  in  one  department  of 
science  is  necessary  for  the  study  of  another  ; 
indeed,  the  phenomena  which  need  for  their  inter- 
pretation the  methods  of  two  branches  of  science 
have  proved  often  the  most  fruitful  field  of  inquiry. 

For  reasons  such  as  these  it  has  been  thought 


vi  PREFACE 

possible  that  a  short  account  of  some  of  the  im- 
portant investigations  now  being  carried  on  in 
the  physical  laboratories  of  the  world  might 
prove  useful  to  students  of  science  in  general ; 
while  it  is  hoped  that,  by  treating  the  subjects 
as  far  as  possible  without  technical  language, 
the  book  may  also  appeal  to  those  who,  with 
little  definite  scientific  training,  are  interested 
in  the  more  important  conclusions  of  scientific 
thought. 

The  writer  has  been  fortunate  in  his  surround- 
ings, where  the  knowledge  and  insight  of  one 
worker  are  freely  and  ungrudgingly  placed  at  the 
service  of  another  in  the  day  of  his  need.  In  the 
present  undertaking  he  records  gratefully  the  help 
of  several  friends  who  have  read  the  proof  sheets 
of  the  parts  dealing  with  subjects  with  which 
their  names  are  closely  associated.  Mr.  F.  H. 
Neville  criticised  the  chapter  on  The  Philosophical 
Basis  of  Physical  Science,  and  that  on  Fusion  and 
Solidification.  Lord  Berkeley  read  the  account 
of  The  Problems  of  Solution.  Professor  J.  J. 
Thomson  saw  the  manuscript  of  the  original 
article  on  which  is  founded  the  chapters  on 
Conduction  of  Electricity  through  Gases  and 


PREFACE  vii 

Radio-Activity.  Professor  Larmor  revised  the 
account  of  Atoms  and  ./Ether,  while  Mr.  H.  F. 
Newall  read  the  chapter  on  Astro-Physics.  For 
this  assistance  the  writer  expresses  his  cordial 
gratitude.  He  wishes  especially  to  thank  his  wife 
for  continual  correction  both  of  the  manuscript 
and  of  the  proof  sheets,  and  his  sister  for  help 
with  the  index. 

The  editor  of  the  Quarterly  Review  has  kindly 
allowed  use  to  be  made  of  the  article  on  Matter 
and  Electricity  which  appeared  in  January  1904. 
Professor  George  E.  Hale  was  good  enough  to 
permit  some  of  his  photographs  of  the  sun  to  be 
reproduced,  while,  for  other  illustrations,  acknow- 
ledgments are  due  to  the  Royal  Society,  to  Mr. 
Heycock  and  Mr.  Neville,  and  to  Mr.  G.  T.  Beilby. 
Lord  Kelvin  kindly  sent  a  signed  portrait,  and  Pro- 
fessor J.  J.  Thomson  allowed  the  use  of  a  repro- 
duction of  Mr.  Arthur  Hacker's  admirable  painting, 
which  now  hangs  in  the  Cavendish  Laboratory. 

In  spite  of  the  generous  help  he  has  received, 
the  author  is  sadly  conscious  of  the  difficulty  of 
his  task.  Although  the  development  of  physical 
science  is  one  of  the  most  powerful  activities  of 
our  time,  a  knowledge  of  its  aims,  methods,  and 


viii  PREFACE 

results  has  not  yet  been  recognised  as  a  necessary 
part  of  an  English  liberal  education.  To  give 
a  popular  exposition  of  results,  especially  when 
there  is  an  obvious  technical  application,  is  easy  ; 
to  enable  a  non-scientific  mind  to  follow  and 
appreciate  the  methods  by  which  the  results  are 
reached  is  supremely  difficult.  But  in  science 
methods  are  usually  more  important  than  results, 
while  a  superficial  acquaintance  with  results  with- 
out an  underlying  knowledge  of  method  is  useless, 
or  worse  than  useless. 

In  the  possibility  of  treating  the  wider  and 
deeper  generalisations  of  natural  science  as  fit 
subject-matter  for  current  thought  and  literature, 
the  writer  has  a  profound  belief.  Whether  the 
failure  to  secure  such  treatment  has  been  due  to 
lack  of  adequate  exposition,  or  to  some  radical 
defect  in  the  training  of  the  nation,  is  a  difficult 
and  grave  problem  ;  but,  until  the  point  of  view 
has  been  altered,  it  is  perhaps  hopeless  to  look 
for  a  proper  understanding  of  the  scientific  spirit 
and  of  scientific  method  even  among  the  more 
educated  portion  of  the  community.  For  the  pre- 
sent, the  man  of  science  must  perforce  occupy 
a  more  technical  and  isolated  position  than  the 


PREFACE  ix 

student  of  history  or  the  lover  of  art.  From  the 
point  of  view  of  the  man  of  science,  to  break 
down  this  isolation  would  be,  at  best,  but  sorry 
kindness  ;  but,  from  a  wider  point  of  view,  for 
the  good  of  the  nation  and  of  mankind,  a  more 
general  acceptance  of  a  share  in  the  impersonal 
open-minded  search  for  truth,  which  is  the  essence 
of  science,  is  ardently  to  be  desired. 

With  some  such  thoughts  as  these,  the  writer 
sends  forth  the  following  pages. 

CAMBRIDGE,  June  25,  1904. 


CONTENTS 


PAGE 

INTRODUCTION I 

CHAPTER  I 

THE   PHILOSOPHICAL    BASIS    OF   PHYSICAL   SCIENCE        .          II 

CHAPTER  II 

THE    LIQUEFACTION     OF    GASES     AND    THE   ABSOLUTE 

ZERO    OF    TEMPERATURE 45 

CHAPTER  III 

FUSION   AND    SOLIDIFICATION       .....         78 

CHAPTER  IV 

THE  PROBLEMS  OF  SOLUTION  .     .     .     .     .   108 

CHAPTER  V 

THE   CONDUCTION    OF   ELECTRICITY   THROUGH   GASES         148 

CHAPTER  VI 

RADIO-ACTIVITY 197 


xii  CONTENTS 

CHAPTER  VII 

PAGE 

ATOMS   AND   AETHER 244 

CHAPTER  VIII 

ASTRO-PHYSICS 293 


INDEX 


•  •      338 


PHYSICAL     SCIENCE 


"  Not  clinging  to  some  ancient  saw  ; 

Not  mastered  by  some  modern  term  ; 
Not  swift  nor  slow  to  change,  but  firm  : 
And  in  its  season  bring  the  law." 

—TENNYSON. 

IN  the  great  advance  of  recent  years,  Physical 
Science  has  developed  chiefly  in  two  directions. 
Although  these  movements  have  been  contempo- 
raneous, it  is  interesting  to  note  that  the  methods 
employed  by  the  two  schools  of  research  are,  to 
some  extent,  the  expression  of  opposite  tendencies. 
On  the  one  hand,  we  see  the  growth  of  the  study 
of  the  phenomena  of  equilibrium  in  physical  and 
chemical  systems ;  this  growth  is  due  to  the  thermo- 
dynamic  methods  founded  chiefly  on  the  great 
work  of  the  late  Willard  Gibbs,  of  Yale  University 
in  the  United  States.  On  the  other  hand,  our 
knowledge  of  the  mode  of  the  conduction  of  elec- 
tricity through  gases  has  been  extended,  mainly 
by  the  efforts  of  J.  J.  Thomson,  Professor  of 
Experimental  Physics  at  Cambridge,  and  of  the 
band  of  workers  trained  by  him  in  the  Cavendish 

A 


2  PHYSICAL  SCIENCE 

Laboratory.  Of  late  years  students  from  almost 
all  civilised  countries  have  come  to  Cambridge  as 
to  the  centre  of  this  branch  of  physical  research, 
and  many  of  them  are  now  carrying  forward  their 
investigations  elsewhere,  by  methods  learnt  in  the 
University  of  Newton,  Clerk-Maxwell,  and  Stokes. 
As  we  shall  see  in  the  following  pages,  the 
work  of  this  school  of  physicists  is  undertaken  and 
interpreted  by  the  aid  of  atomic  and  molecular 
conceptions.  The  theory  of  the  conduction  of 
electricity  through  liquids,  based  originally  on 
the  work  of  Faraday,  and  slowly  matured  during 
the  last  half-century  by  Hittorf,  Kohlrausch, 
Arrhenius,  and  many  others,  had  accustomed 
our  minds  to  the  conception  of  electric  con- 
duction by  means  of  the  motion  of  charged 
particles,  called  by  Faraday  "  ions  " — the  travellers. 
Each  ion  consists  of  an  atom,  or  group  of  atoms,  of 
the  substance  in  solution,  associated  with  a  positive 
or  negative  electric  charge;  it  moves  through 
the  liquid  under  the  action  of  an  applied  electric 
force,  and  gives  up  its  charge  to  the  electrode — 
that  is,  the  terminal  by  which  the  current  enters 
or  leaves  the  liquid.  The  conduction,  instead 
of  being  conceived  as  a  river  flowing  uniformly, 
must  figuratively  be  represented  as  taking  place 
by  the  passage  of  discrete  quantities  of  electricity; 
in  much  the  same  way  as  water  is  sometimes 


INTRODUCTION  3 

carried  from  a  lake  to  a  burning  house  by  means 
of  a  chain  of  bucket-bearers. 

By  the  application  of  similar  conceptions,  the 
passage  of  electricity  through  gases  has  received 
a  convincing  explanation.  Differences  appear,  but 
the  fundamental  ideas  are  the  same  in  the  two 
branches  of  the  science  of  electrolytic  conduction. 
It  is,  however,  in  the  newer  side  of  the  subject 
that  the  most  striking  results  have  been  obtained. 
Electrolysis  in  liquids  had  suggested  the  concep- 
tion of  ultimate  units  of  electricity  —  atoms  of 
electricity,  analogous  to  the  atoms  of  matter. 
Gaseous  conduction  enabled  these  electric  atoms 
to  be  isolated,  separated  from  their  attendant 
material  atoms,  and  studied  independently. 

Great  has  been  the  revelation  which  followed. 
The  isolated  atoms  of  negative  electricity  —  the 
electrons,  as  they  have  been  named  by  Stoney — 
have  been  identified  by  the  work  of  Thomson, 
Lorentz,  and  Larmor,  with  the  physical  basis  of 
matter,  with  the  corpuscles,  or  sub-atoms,  by 
means  of  which,  combined  in  varying  numbers 
and  in  different  arrangements,  are  composed 
the  chemical  atoms,  for  long  taken  as  ultimate 
indivisible  units. 

Farther  light  has  been  thrown  on  these  dark 
places  by  the  remarkable  series  of  discoveries 
through  which  M.  and  Mme.  Curie  and  other 


4  PHYSICAL  SCIENCE 

chemists  have  given  us  the  radio-active  elements 
such  as  radium,  and  the  parallel  series  in  which 
Rutherford  has  interpreted  their  properties  as 
due  to  the  disintegration  of  their  atoms,  as,  one 
after  another,  those  atoms  break  down,  and  are 
transmuted  into  other  substances. 

Throughout  these  investigations  we  deal  with 
atomic  and  molecular  conceptions  in  an  extreme 
form.  We  look  even  within  the  atom,  and 
examine  its  internal  structure ;  we  trace  the  cor- 
puscles or  electrons  flying  round  in  their  orbits,  as 
we  watch  the  planets  swinging  round  the  sun. 

It  is  remarkable  that,  in  the  other  branch 
of  Physical  Science  in  which  simultaneous 
progress  has  been  most  striking,  the  methods 
chiefly  employed  have  enabled  us  to  dispense 
altogether  with  atomic  and  molecular  theories. 

At  the  basis  of  the  theory  of  physical  and 
chemical  equilibrium  lies  Lord  Kelvin's  great 
principle  of  the  dissipation  of  energy.  While  the 
total  amount  of  energy  in  an  isolated  system  is 
unchanging  and  unchangeable,  that  energy  is 
tending  always  to  become  less  available  for  the 
performance  of  useful  work.  The  availability  of 
the  energy  tends  continually  to  become  less.  It 
follows  that  permanent  equilibrium  can  only  be 
attained  when  the  limit  has  been  reached  and 
the  availability  is  a  minimum.  Such  a  theorem 


INTRODUCTION  5 

is  independent  of  molecular  hypotheses  ;  in  fact, 
it  expressly  disclaims  such  hypotheses,  for,  as 
Maxwell  showed,  the  chance  collisions  of  the 
individual  molecules  in  a  gas  will  lead  to  differ- 
ing molecular  velocities,  and  to  a  concentration 
of  energy  in  the  fast-moving  molecules.  If  we 
could  follow  the  motions  of  the  individual  mole- 
cules, and  separate  the  fast  from  the  slow,  we 
could  use  this  energy.  The  principle  of  dissipa- 
tion, therefore,  only  holds  while  we  are  obliged, 
as  of  course  is  always  the  case  in  practice, 
to  deal  with  molecules  statistically  and  in  the 
aggregate. 

The  principles  thus  applied  to  isolated  systems 
have  been  extended  to  the  visible  universe. 
Predictions  have  been  made  that  ultimately  the 
energy  of  the  universe  will  become  completely 
unavailable,  and  will  settle  down  into  the  energy 
of  heat,  uniformly  distributed.  But  this  final 
sleep  of  the  universe  depends  on  the  assumptions 
that  the  universe  is  an  isolated  system,  finite  in 
extent,  and  that  no  process  of  molecular  concen- 
tration of  energy,  such  as  was  imagined  by 
Maxwell,  is  going  on  anywhere  throughout  the 
depths  of  time  and  space. 

A  more  restricted,  though  more  fruitful,  appli- 
cation of  the  dissipation  principle  enabled  Helm- 
holtz,  and,  in  a  much  more  general  manner, 


6  PHYSICAL  SCIENCE 

Willard  Gibbs,  to  place  on  a  firm  footing  the 
theory  of  non-isolated  but  isothermal  systems — 
systems,  that  is,  maintained  at  a  uniform  and  con- 
stant temperature  by  the  gain  or  loss  of  external 
heat.  The  external  work  which  such  a  system  can 
perform,  by  means  of  a  reversible  change  at  con- 
stant temperature,  tends  to  a  minimum,  and  the 
system  is  in  permanent  equilibrium  when,  and 
when  only,  this  available  or  free  energy,  as  it  is 
called,  becomes  as  small  as  possible.  By  this  sole 
principle,  Willard  Gibbs  developed  the  complete 
theory  of  chemical  and  physical  equilibrium  ; 
as  Professor  Larmor  says,  his  "  monumental 
memoir  made  a  clean  sweep  of  the  subject ; 
and  workers  in  the  modern  experimental  science 
of  physical  chemistry  have  returned  to  it  again 
and  again  to  find  their  empirical  principles  fore- 
casted in  the  light  of  pure  theory,  and  to  derive 
fresh  inspiration  for  new  departures." 

Simultaneously  with  the  development  of  ex- 
perimental research  along  the  two  lines  we  have 
indicated,  has  arisen  afresh  an  interest  in  and 
inquiry  into  the  philosophic  basis  on  which  is 
built  the  whole  magnificent  structure  of  modern 
science.  How  far  is  that  basis  secure  ?  Are  the 
conceptions  of  science  life-like  pictures  of  any 
fundamental  reality  behind  the  phenomena  which 
alone  our  senses  can  apprehend  ?  Such  questions 


INTRODUCTION  7 

have  occupied  periodically  the  ablest  minds  of 
certain  epochs  of  history,  though  in  the  attempts 
to  find  answers  no  such  general  consensus  of 
opinion  has  been  reached  as  we  see  within  the 
building  of  science  itself.  Granted  the  security  of 
the  foundations,  the  edifice  seems  designed  on  a 
consistent  plan,  for  the  relations  of  its  parts  pre- 
sent themselves  similarly  to  all  minds  competent 
to  judge. 

The  philosophy  of  science  is  intimately  con- 
nected with  its  history;  and  interest  has  been 
stimulated  afresh  in  the  philosophical  problems 
involved  in  physical  conceptions  by  the  publica- 
tion of  Mach's  great  work  on  the  Science  and 
History  of  Mechanics.  To  many  that  book  has 
put  new  life  into  the  subject  treated  in  its  pages, 
and  has  led  to  a  more  careful  consideration  of 
the  fundamental  conceptions  of  natural  science 
in  general. 

In  the  following  pages  an  attempt  will  be 
made  first  to  consider  the  philosophic  foundations 
of  physics,  and  then  to  trace  some  of  the  more 
important  developments  of  the  experimental  in- 
vestigations for  which  the  last  few  years  have 
been  remarkable. 

The  study  of  physical  equilibrium — the  equi- 
librium between  different  states  or  phases,  solid, 
liquid,  and  gaseous,  of  the  same  substance — 


8  PHYSICAL  SCIENCE 

naturally  opens  with  the  consideration  of  the 
relations  between  the  different  states  of  pure 
chemical  elements  and  compounds.  Here,  the 
most  striking  recent  work  is  the  liquefaction  of 
air  and  hydrogen,  with  which  the  name  of  Dewar 
most  prominently  must  be  associated. 

Next  we  turn  to  mixtures,  and  the  fusion  and 
solidification  of  solutions  and  alloys  claim  our 
attention.  The  microscopic  analysis  of  metals, 
when  elucidated  by  the  theory  of  equilibrium, 
has  had  far-reaching  influence  on  the  practical 
arts  of  metallurgy. 

Then  are  considered  the  problems  of  solution 
in  general,  without  restriction  to  conditions  of 
equilibrium.  Now,  for  the  first  time,  we  come 
in  contact  with  electrical  phenomena ;  and  the 
theory  of  ionic  conduction  throws  light,  not  only 
on  the  nature  of  electrolytic  solutions,  but  on 
many  physiological  questions  of  vital  interest. 

A  natural  step  leads  from  the  conduction  of 
electricity  in  liquids  to  its  conduction  in  gases, 
and,  on  our  stage,  the  ion  is  joined  by  the  cor- 
puscle or  electron.  The  dream  of  the  old  philo- 
sophers of  a  common  basis  for  matter  is  realised 
by  experimental  investigation. 

Arising  from  these  experiments  and  their  inter- 
pretation comes  the  theory  of  radio-activity,  the 
modern  equivalent  of  the  imagined  transmutation 


INTRODUCTION  9 

of  the  mediaeval  alchemist.  Though  the  changes 
are  beyond  our  control,  we  see  and  measure 
the  gradual  evolution  and  disintegration  of  the 
chemical  elements,  and  draw  on  the  energy  stored 
within  the  atoms  themselves. 

The  vibrations  of  electro-magnetic  systems  pro- 
duce the  aethereal  waves  now  used  in  wireless  tele- 
graphy, and  the  vibrations  of  atomic  systems  give 
rise  to  light.  Thus  atoms  must  be  intimately  related 
to  the  luminiferous  aether,  and  light  to  electro- 
magnetic phenomena.  Corpuscles  or  electrons, 
too,  cry  aloud  for  a  physical  explanation  in  terms 
of  aethereal  conceptions  ;  and  Larmor's  idea  of  an 
electron  as  a  centre  of  intrinsic  aethereal  strain 
gives  us  a  possible  formulation  of  the  subject, 
and,  in  some  form  or  other,  seems  now  to  hold 
the  field. 

Finally,  we  pass  to  the  bearing  of  all  this  new 
knowledge  on  cosmical  problems.  Physics  is 
rapidly  annexing  the  domain  of  astronomy,  as  it 
has  already  invaded  the  realms  of  chemistry  and 
biology.  By  the  aid  of  the  spectroscope  we 
examine  the  chemical  nature  of  the  sun  and  stars, 
we  measure  the  rates  of  their  motions  and  re- 
volutions, and  obtain  data  from  which  we  may 
speculate  about  their  origin,  development,  and 
decay.  From  the  internal  structure  of  the  atom 
to  the  majestic  progress  of  the  suns,  the  investiga- 


io  PHYSICAL  SCIENCE 

tions  of  Physical  Science  are  surely  and  continu- 
ously gaining  new  knowledge  for  mankind. 


We  scatter  the  mists  that  enclose  us, 

Till  the  seas  are  ours  and  the  lands, 
Till  the  quivering  aether  knows  us, 

And  carries  our  quick  commands. 
From  the  blaze  of  the  sun's  bright  glory 

We  sift  each  ray  of  light, 
We  steal  from  the  stars  their  story 

Across  the  dark  spaces  of  night. 

But  beyond  the  bright  search-lights  of  science, 

Out  of  sight  of  the  windows  of  sense, 
Old  riddles  still  bid  us  defiance, 

Old  questions  of  Why  and  of  Whence. 
There  fail  all  sure  means  of  trial, 

There  end  all  the  pathways  we've  trod, 
Where  man,  by  belief  or  denial, 

Is  weaving  the  purpose  of  God. 


CHAPTER    I 

THE    PHILOSOPHICAL    BASIS    OF    PHYSICAL    SCIENCE 

"  Homo,  naturae  minister  et  interpres,  tantum  facit  et  intelligit 
quantum  de  naturae  ordine  re  vel  mente  observaverit.  .  .  .  Natura 
enim  non  nisi  parendo  vincitur.  .  .  ." — BACON,  Novum  Organum. 

THE  mind  of  man,  learning  consciously  and  uncon- 
sciously lessons  of  experience,  gradually  constructs 
a  mental  image  of  its  surroundings — as  the  mariner 
draws  a  chart  of  strange  coasts  to  guide  him  in 
future  voyages,  and  to  enable  those  that  follow  after 
him  to  sail  the  same  seas  with  ease  and  safety.  The 
chart  may  be  drawn  to  scale ;  it  may  be  consistent 
with  itself  and  serve  its  purpose — but  it  only  repre- 
sents the  earth's  surface  in  one  limited  and  con- 
ventional manner ;  it  does  not  give  a  life  -  like 
picture  of  the  original  in  the  same  sense  as  does 
a  photograph  or  a  painting.  So  it  is  with  the 
ideas  that  our  minds  conceive  of  the  world  around 
us,  and  with  the  model  of  that  world  which  our 
minds  construct.  And  this  analogy  may  serve  to 
interpret  to  us  our  attitude  towards  the  concep- 
tion that  the  human  race  has  formed  of  the  world 
we  live  in.  If  the  model  be  consistent,  if  the  various 


12  PHYSICAL  SCIENCE 

parts  and  aspects  of  it  do  not  fail  to  correspond 
with  each  other,  it  serves  the  double  purpose 
of  introducing  order  into  what  would  otherwise 
be  mental  confusion,  and  of  helping  us  to  make 
systematic  use  of  the  resources  of  Nature. 

Confronted  with  the  mystery  of  the  Universe,  we 
are  driven  to  ask  if  the  model  our  minds  have 
framed  at  all  corresponds  with  the  reality  ;  if, 
indeed,  there  be  any  reality  behind  the  image.  Such 
a  question  is  a  proper  study  of  philosophy,  but  need 
not  necessarily  be  answered  for  the  model  to  be 
made  or  used.  The  whole  problem  mankind  has  to 
face  undoubtedly  includes  this  fundamental  ques- 
tion of  the  ultimate  nature  of  reality,  which  would 
enter  into  a  complete  explanation  of  every  fact,  even 
of  those  which  we  regard  as  the  simplest.  This 
general  aspect  of  the  problem  is  the  subject  of  that 
branch  of  philosophy  known  as  Metaphysics.  But, 
if  we  confine  our  attention  to  the  phenomena  which 
our  senses  apprehend,  and,  thus  restricting  our 
inquiry,  examine  our  mental  picture  of  Nature  and 
the  relation  of  its  parts  to  each  other,  testing  their 
correspondence  or  want  of  correspondence,  we  are 
studying  Natural  Science.  The  limitation  indicated 
has  not  always  been  observed,  and  the  name  of 
Natural  Philosophy  survives  to  remind  us  that 
Natural  Science  is  but  one  part  of  the  whole  of 
conceivable  knowledge. 


THE  PHILOSOPHICAL  BASIS          13 

The  problem  of  Metaphysics  is  of  much  greater 
difficulty  than  that  of  Natural  Science.  Hence, 
Natural  Science  has  only  begun  to  make  rapid 
progress  since  its  separation  from  Metaphysics. 
Despite  the  closest  attention  of  the  acutest  intellects 
since  the  age  of  Greece,  no  general  consensus  of 
opinion  has  been  reached  by  metaphysicians. 
Materialism,  Dualism,  Idealism,  inconsistent  views 
of  the  nature  of  reality,  are  all  of  them  still  held  by 
competent  philosophers  : 

"  Myself  when  young  did  eagerly  frequent 

Doctor  and  saint,  and  heard  great  argument 
About  it  and  about  :  but  evermore 
Came  out  by  the  same  door  where  in  I  went." 

The  slow  and  laborious  methods  of  observation 
and  experiment  have  been  pursued  from  the  earliest 
times  for  purposes  of  common  life  and  technical 
industry.  They  were  first  considered  philosophi- 
cally though  inadequately  by  Bacon,  and  by  their 
help  a  firm  ground  has  been  obtained  for  the 
edifice  of  Natural  Science.  In  contrast  with  the 
results  of  Metaphysics,  a  general  consensus  of 
scientific  opinion  upon  fundamental  points  has  been 
obtained.  No  physicist  of  repute  doubts  the  validity, 
within  narrow  limits  of  error,  of  Newton's  law  of 
gravity,  or  of  the  principle  of  the  conservation  of 
energy. 

But  observation  and  experiment  can  be  directed 


14  PHYSICAL  SCIENCE 

only  to  the  examination  of  our  conceptions.  In 
this  way  we  gain  materials  for  the  construction  and 
examination  of  the  mind's  model  of  reality ;  we 
do  not  touch  reality  itself.  If  this  be  doubted,  we 
must  reflect  that  we  can  apprehend  the  results  of 
experiment  through  our  senses  alone.  Though,  for 
instance,  the  galvanometer  seems  at  first  to  supply 
us  with  a  new  electrical  sense,  on  further  thought 
we  see  that  it  merely  translates  the  unknown  into  a 
language  our  sense  of  sight  can  appreciate,  as  a 
spot  of  light  moves  over  a  scale.  It  is  possible 
that  Philosophy  may  take  into  account  knowledge 
which  reaches  us  by  means  other  than  the  senses. 
Intuitions,  fundamental  assumptions,  mental  pro- 
cesses generally,  doubtless  have  an  external  aspect, 
and  may  be  studied  by  the  science  of  Psycho- 
physics,  but  they  may  have  also  another  aspect  in 
their  internal  relations  to  consciousness.  Here 
they  can  be  examined  by  Metaphysics.  But  we 
can  only  study  Nature  through  our  senses — that  is, 
we  can  only  study  the  model  of  Nature  that  our 
senses  enable  our  minds  to  construct;  we  cannot 
decide  whether  that  model,  consistent  though  it 
be,  represents  truly  the  real  structure  of  Nature; 
whether,  indeed,  there  be  any  Nature  as  an  ultimate 
reality  behind  its  phenomena. 

In  emphasising  the  essential  distinction  between 
Natural  Science  and  Metaphysics,  we  must  not  sup- 


THE  PHILOSOPHICAL  BASIS          15 

pose  that  the  results  of  Natural  Science  have  no 
metaphysical  import.  The  possibility  of  putting  to- 
gether a  consistent  mental  model  of  phenomena  is  a 
valid  metaphysical  argument  in  favour  of  the  view 
that  a  consistent  reality  underlies  those  phenomena, 
and  that  the  reality  is  represented  with  more  or  less 
faithfulness  by  the  mental  picture  we  have  pieced 
together.  Such  an  argument  must  carry  great 
weight,  and  may,  perhaps,  be  considered  conclusive; 
but  it  is  a  metaphysical  argument,  not  one  with 
which  Natural  Science  is  concerned  directly.  In 
framing  and  attempting  to  answer  her  own  deeper 
questions,  Metaphysics  uses  the  results  of  Natural 
Science,  as  indeed  of  all  other  branches  of  inquiry. 
But  this  does  not  make  Natural  Science  a  branch 
of  Metaphysics,  or  remove  the  essential  difference 
between  the  subjects  of  the  two  studies. 

The  object  of  Natural  Science,  then,  is  to  fit 
together  a  consistent  and  harmonious  model  which 
shall  represent  to  our  minds  the  phenomena  which 
act  on  our  senses.  We  need  not  fear  that  this 
limitation  will  lower  the  dignity  or  circumscribe 
unduly  the  extent  of  our  inquiries.  Whether  we 
look  inwards  or  outwards  the  complexity  of  Nature 
seems  boundless  : 

"  Boundless  inward  in  the  atom  ;  boundless  outward  in 
the  whole." 


16  PHYSICAL  SCIENCE 

The  more  we  learn,  the  more  various  and  intricate 
are  the  new  avenues  of  research  which  open  before 
us.  As  has  been  well  said,  the  larger  grows  the 
sphere  of  knowledge,  the  greater  becomes  its  area 
of  contact  with  the  unknown. 

So  complex  would  be  an  entire  mental  picture 
of  phenomena,  that  divisions  of  Natural  Science 
have  arisen,  each  of  them  tending  more  and  more 
to  demand  the  exclusive  attention  of  the  specialist. 
These  divisions  are  purely  arbitrary ;  they  have 
arisen  partly  from  differences  in  methods  of  in- 
quiry, partly  from  historical  reasons.  Moreover, 
they  are  variable,  and  are  shifted  from  time  to  time 
according  to  the  needs  of  each  department  and 
the  prevalent  direction  of  inquiry,  while  new 
divisions  may  spring  into  existence. 

The  different  sciences  are  not  even  parts  of  a 
whole ;  they  are  but  different  aspects  of  a  whole, 
which  essentially  has  nothing  in  it  corresponding 
to  the  divisions  we  make ;  they  are,  so  to  speak, 
sections  of  our  model  of  Nature  in  certain  arbitrary 
planes,  cut  in  directions  to  suit  our  convenience. 
Thus  a  nerve-impulse  may  be  considered  in  a  psy- 
chological aspect,  a  physiological  aspect,  or  a 
physical  aspect.  Even  these  divisions  may  be  sub- 
divided ;  the  physics  of  the  nerve  impulse  may  be 
studied  first  from  the  electrical  side  by  investigat- 
ing the  electric  currents  that  accompany  it,  and 


THE   PHILOSOPHICAL  BASIS  17 

then  from  the  mechanical  side,  by  correlating  the 
electrical  currents  with  the  movements  of  matter  that 
simultaneously  occur.  No  one  of  these  aspects  of 
the  phenomenon  is  essentially  more  fundamental 
than  any  other,  and  the  conviction  at  one  time  pre- 
valent, and  even  now  by  no  means  uncommon, 
that  a  complete  mechanical  explanation  of  every 
phenomenon  is  possible  and  fundamental,  seems 
merely  an  unphilosophical  fallacy.  Its  origin  is  to 
be  sought  in  the  historical  fact  that  the  section 
known  as  mechanics  was  the  earliest  of  the  physical 
sciences,  and  that  its  methods  and  conclusions  are 
fairly  intelligible  to  the  ordinary  man,  and,  in  their 
elements,  essential  to  his  daily  life.  The  science  of 
mechanics  has  been  more  fully  developed  from  its 
experimental  basis  by  the  methods  of  mathematical 
deduction  than  any  other  branch  of  Natural  Know- 
ledge, and  mankind  has  hence  come  to  believe 
that  it  is  essentially  simpler  and  nearer  reality. 
But  in  truth  it  is  no  more  fundamental  than  elec- 
tricity, and,  as  we  shall  see  in  the  following  pages, 
there  is  a  growing  tendency  in  modern  thought  to 
conceive  matter  itself  as  an  electrical  manifestation. 
Again,  it  is  sometimes  argued  that  mechanics  is 
the  fundamental  science  because  its  extension  is 
universal,  while  that  of  physiology,  for  example,  is 
not.  The  contraction  of  a  muscle  has  clearly  a 
mechanical  aspect,  while  the  fall  of  a  stone  to  the 


i8  PHYSICAL  SCIENCE 

earth  has  nothing  to  do  with  physiology.  Even 
a  thought,  from  one  side  purely  a  psychological 
phenomenon,  may  have  a  mechanical  aspect  if  we 
could  trace  the  physical  changes  in  the  brain  which 
accompany  it,  whereas,  it  may  be  said,  the  expansion 
of  steam  in  an  engine  has  no  psychological  sig- 
nificance. Such  considerations  certainly  indicate 
that  the  arbitrary  plane  cut  through  our  solid 
model  of  the  universe  by  mechanical  science  is  cut 
in  such  a  place  that  it  traverses  a  large  part  of  the 
model — a  larger  part,  perhaps,  than  any  other  section 
which  has  yet  been  cut.  It  does  not  follow,  how- 
ever, that  it  cuts  through  the  whole ;  still  less  that 
a  plane  section  can  represent  fully  a  solid  model. 
Thus  the  argument  that,  because  of  its  wide  ex- 
tension, mechanics  has  some  fundamental  signifi- 
cance is  seen  to  be  a  fallacy.  It  may  be  prima 
inter  pares  of  the  natural  sciences,  but  nothing 
more.  To  go  even  further  than  this,  as  has  some- 
times been  done,  and  to  suppose  that  the  ultimate 
nature  of  reality  is  the  same  essentially  as  our  idea 
of  a  single  arbitrary  section,  cut  through  an 
imaginary  model  of  it,  seems  only  to  need  stating 
in  these  terms  to  be  disbelieved. 

As  Mach  has  well  said,  "Physical  Science  does 
not  pretend  to  be  a  complete  view  of  the  world; 
it  simply  claims  that  it  is  working  towards  such  a 
complete  view  in  the  future.  The  highest  philo- 


THE   PHILOSOPHICAL  BASIS  19 

sophy  of  the  scientific  investigator  is  precisely 
this  toleration  of  an  incomplete  conception  of  the 
world  and  the  preference  for  it  rather  than  for  an 
apparently  perfect  but  inadequate  conception." 

When  the  experimental  study  of  nature  was  new, 
when  man  first  caught  a  glimpse  of  order  in  the 
multiplicity  of  phenomena,  such  a  view  of  the  all- 
comprehending  character  of  mechanical  science 
seemed  just.  Let  us  again  listen  to  Mach  : — 

"The  French  encyclopaedists  of  the  eighteenth 
century  imagined  they  were  not  far  from  a  final 
explanation  of  the  world  by  physical  and  mechani- 
cal principles ;  Laplace  even  conceived  a  mind 
competent  to  foretell  the  progress  of  nature  for  all 
eternity,  if  but  the  masses,  their  positions,  and 
initial  velocities  were  given.  In  the  eighteenth 
century,  this  joyful  over-estimation  of  the  scope  of 
the  new  physico-mechanical  ideas  is  pardonable. 
Indeed,  it  is  a  refreshing,  noble,  and  elevating 
spectacle  ;  and  we  can  deeply  sympathise  with  this 
expression  of  intellectual  joy,  so  unique  in  history. 
But  now,  after  a  century  has  elapsed,  after  our 
judgment  has  grown  more  sober,  the  world-con- 
ception of  the  encyclopaedists  appears  to  us  as  a 
mechanical  mythology  in  contrast  with  the  animistic 
mythology  of  the  old  religions.  Both  views  contain 
undue  and  fantastical  exaggerations  of  an  incom- 
plete perception.  Careful  physical  inquiry  will 


20  PHYSICAL  SCIENCE 

lead,  however,"  to  a  more  complete  philosophy. 
"The  direction  in  which  this  enlightenment  is  to  be 
looked  for,  as  the  result  of  long  and  painstaking 
research,  can  of  course  only  be  surmised.  To 
anticipate  the  result,  or  even  to  attempt  to  intro- 
duce it  into  any  scientific  investigation  of  to-day, 
would  be  mythology,  not  science." 

Physical  Science,  then,  the  subject  of  the  present 
work,  is  merely  one  aspect  from  which  we  may 
agree  to  look  at  the  model  of  Nature  that  our  minds 
construct.  It  ignores  the  biological  standpoint, 
from  which  phenomena  are  regarded  in  their  bear- 
ing on  life ;  it  ignores  the  psychological  standpoint, 
from  which  they  are  studied  in  relation  to  mind. 
With  these  limitations,  let  us  see  what  kind  of 
model  of  Nature  we  are  led  to  build. 

The  ideas  of  length  and  time  may  be  regarded  as 
primary — length  as  the  simplest  form  of  space  con- 
ception, time  as  a  recognition  of  sequence  in  our 
states  of  consciousness.  One  of  the  earliest  ad- 
vances in  exact  science  was  the  power  of  counting 
and  the  resultant  method  of  expressing  quantities 
as  numbers.  In  spite  of  its  essential  nature,  the 
capacity  for  so  doing  is  by  no  means  innate ;  nor 
is  it  even  yet  properly  developed  among  all  the 
races  inhabiting  this  globe.  In  order  to  measure 
quantities,  it  is  necessary  to  choose  or  invent  some 


THE  PHILOSOPHICAL  BASIS  21 

unit,  and  then  to  count  the  number  of  times  that 
unit  is  comprised  in  the  quantity  to  be  measured. 
In  civilised  countries  the  unit  of  length  is  taken 
as  the  length  between  two  marks  on  a  certain 
standard  metallic  bar.  In  England  there  is  a 
standard  yard,  and  in  France  a  standard  metre. 
In  fact,  both  these  units  are  arbitrarily  selected  for 
their  convenience,  though  the  original  idea  of  the 
metre  was  derived  from  a  connection  with  the 
supposed  dimensions  of  the  earth. 

Like  the  unit  of  length,  the  unit  of  time  is  arbi- 
trary, and  ultimately  rests  on  a  measure  of  our 
sequence  of  consciousness.  Again  we  have  to 
choose  some  arbitrary  unit,  which,  in  this  case, 
should  always  contain,  under  similar  conditions, 
a  similar  amount  of  human  consciousness.  For 
purposes  of  the  convenience  of  daily  life  the  ob- 
vious unit  to  select  is  the  day,  while  the  sequence  of 
the  seasons  suggests  another  equally  arbitrary  unit 
— the  year.  The  exact  relation  between  these  two 
units  can  only  be  determined  by  careful  astrono- 
mical observation.  Wrong  determination  and  con- 
sequent re-determination  have  led  from  time  to 
time  to  necessary  changes  of  calendar ;  while  the 
partial  adoption  of  these  changes  has  resulted  in 
the  inconvenient  differences  of  date  in  vogue  among 
the  various  nations.  That  the  units  of  time  cannot 
be  regarded  as  essentially  fixed  and  unalterable  is 


22  PHYSICAL  SCIENCE 

clear  when  we  remember  that  any  friction  on  the 
earth,  such  as  that  of  the  tides,  is  slowly  prolonging 
the  day,  while  resistance  to  the  bodily  motion  of 
the  earth  round  the  sun  would  gradually  alter  the 
length  of  the  year.  Such  changes  may  become 
appreciable  only  after  the  lapse  of  thousands  or 
millions  of  years  ;  but  the  possibility  of  their  oc- 
currence shows  that  our  time-units  are  as  purely 
arbitrary  as  are  those  of  length. 

From  the  conceptions  of  length  and  time,  and 
the  arbitrary  units  chosen  to  measure  them,  may 
be  derived  the  more  complex  ideas  required  for  a 
description  of  motion,  and  the  derived  units  needed 
to  investigate  it  quantitatively.  Thus  velocity  is 
measured  by  the  ratio  of  the  number  of  units 
of  length  to  the  number  of  units  of  time,  while 
acceleration,  or  the  rate  of  change  of  velocity,  is 
measured  by  the  number  of  units  of  velocity  gained 
or  lost  per  unit  of  time.  These  relations  are  ex- 
pressed by  saying  that  the  dimensions  of  the  unit 
of  velocity  are  L/T,  while  those  of  the  unit  of 
acceleration  are  v/T  or  L/T2. 

With  metaphysical  theories  of  matter,  Physical 
Science  has  no  direct  concern ;  and  mechanics,  at 
any  rate,  deals  only  with  matter  as  that  concep- 
tion, which,  in  our  mental  image  of  phenomena,  is 
always  associated  with  another  and  more  definite 
conception,  that  of  mass.  We  need  not  ask  whether 


THE  PHILOSOPHICAL  BASIS          23 

matter  has  any  objective  existence,  or  whether  our 
conception  of  mass  corresponds  with  any  actual 
property  possessed  by  a  real  thing-in-itself.  Such 
inquiries  are  of  great  interest  and  importance  ;  but 
they  are  metaphysical  inquiries,  not  those  which 
the  physicist,  as  physicist,  must  answer. 

The  conception  of  mass,  as  distinct  from  that 
of  weight,  may  arise  from  the  results  of  our  daily 
experience.  Let  us  suppose,  for  instance,  that  two 
wheels  of  the  same  size,  one  of  wood  and  the  other 
of  iron,  were  mounted  on  axles,  and  were  free  to 
revolve.  When  the  wheels  are  set  in  motion,  the 
weights  do  not  come  into  play,  for  neither  wheel  is 
raised  or  lowered  as  a  whole.  Nevertheless,  a  great 
difference  will  be  felt  if  we  try  to  set  the  two  wheels 
in  motion  suddenly.  It  takes  either  a  much  harder 
push  or  a  much  longer  time  to  produce  a  certain 
velocity  of  rotation  in  the  iron  wheel  than  in  the 
one  made  of  wood.  It  is  this  result  which  leads 
us  to  say  that  the  mass  of  the  iron  wheel  is  the 
greater. 

To  examine  mass  quantitatively,  and  to  obtain 
a  convenient  unit,  more  definite  observation  is 
necessary.  The  mutual  action  of  two  bodies,  as 
examined  by  experiment,  is  such  that  our  descrip- 
tion of  their  relative  motion  becomes  greatly  simpli- 
fied by  assigning  to  each  of  them  a  certain  relative 
number  to  express  a  quantity  which  we  may  term 


24  PHYSICAL  SCIENCE 

its  relative  mass.  Let  us  make  the  two  bodies, 
when  free  to  move,  act  on  each  other  in  any  way, 
excluding  the  possibility  of  rotation,  for  the  sake 
of  simplicity.  Let  us,  for  instance,  connect  them 
by  means  of  a  long,  stretched  elastic  cord,  and 
allow  them  to  move  each  other.  After  the  action 
has  begun,  we  shall  find  that  one  body  is,  in 
general,  moving  faster  than  the  other,  and  that  the 
ratio  of  their  accelerations  is  constant.  The  inverse 
ratio  of  these  accelerations  is  the  measure  of  the 
ratio  between  the  masses  of  the  two  bodies ;  the 
body  with  the  smaller  mass  is  moved  faster  by 
the  mutual  action  than  is  the  body  with  the  greater 
mass. 

We  now  need  only  to  choose  some  mass  as  our 
unit  with  which  to  compare  other  masses,  and  to 
prove  experimentally  that  the  mass  of  a  body  as 
thus  defined  is  a  constant  quantity,  to  complete 
our  preparations  for  using  the  conception  of  mass 
in  our  physical  description  of  observed  phenomena. 

Experience  shows  us  that  we  can  generalise  the 
result  of  our  experiment  on  the  motion  of  the  two 
bodies  connected  with  each  other  by  means  of 
a  string.  We  can  assert  that  no  body  has  an 
acceleration  unless  another  body  is  acting  on  it. 
Thus,  we  cannot  form  a  complete  picture  of  the 
motion  unless  we  consider  both  bodies.  But  it 
is  often  necessary  to  concentrate  our  attention  on 


THE   PHILOSOPHICAL  BASIS          25 

one  of  them,  and  it  is  then  convenient  to  find  some 
quantity  which  correctly  measures  the  effect  of  the 
other  body  on  the  first.  This  quantity  is  not  the 
acceleration,  for  that  depends  on  the  mass  of  the 
moving  body,  but  it  is  the  product  of  the  mass  and 
the  acceleration,  and  is  independent  of  both.  This 
product  completely  records  the  mechanical  effect 
of  the  second  body  ;  we  call  it  a  force,  and  instead 
of  saying  that  one  body  is  acted  on  by  another, 
we  may,  if  more  convenient,  say  that  it  is  acted  on 
by  a  force. 

Together  with  the  primary  conceptions  of  length, 
time,  and  mass,  the  secondary  conception  of  force 
was  employed  by  Newton  in  his  development  of 
mechanical  theory.  A  simultaneous  and  parallel 
development  of  the  science  was  led  by  Huygens, 
who  used  the  conception  which  we  now  call 
work  or  energy  as  a  means  of  co-ordinating  the 
phenomena,  instead  of  stating  them  in  terms  of 
force  as  Newton  did.  Although  it  gave  a  more  in- 
timate insight  into  mechanical  processes,  Newton's 
method  was  perhaps  less  general  than  that  of 
Huygens,  which  often  enables  us  to  pass  directly 
from  a  knowledge  of  the  initial  to  a  prediction  of 
the  final  state  of  a  system,  and  to  avoid  the  diffi- 
culties of  tracing  its  intermediate  operations.  In 
the  history  of  mechanical  science,  now  one  method 
and  now  the  other  has  proved  the  more  useful ; 


26  PHYSICAL  SCIENCE 

and,  in  the  wider  field  of  physics,  the  two  schools 
are  still  represented  by  those  who  seek  to  trace  the 
intimate  processes  of  change  by  means  of  molecular 
theories,  and  by  those  who  rely  on  a  more  general 
presentment,  which  avoids  such  hypotheses  by  the 
use  of  the  principles  of  thermodynamics. 

By  simple  experiments,  such  as  those  described 
above,  the  relative  masses  of  two  reacting  bodies 
may  be  measured  by  the  constant  inverse  ratio  of 
their  accelerations.  It  follows  that  the  product  of 
the  mass  and  the  acceleration  is  the  same  for  the 
two  bodies.  Thus  the  force  which  the  first  body 
exerts  on  the  second  is  the  same  as  the  force  which 
the  second  exerts  on  the  first ;  or,  as  Newton 
expressed  it,  action  and  reaction  are  equal  and 
opposite. 

In  the  ways  we  have  now  considered,  mass  and 
force  may  be  defined  in  a  manner  free  from  all 
metaphysical  subtleties,  and  in  these  senses  alone 
should  they  be  used  in  physical  science.  A  defini- 
tion of  matter  is  not  needed  :  an  inquiry  into 
the  so-called  properties  of  matter  being,  from  the 
physical  point  of  view,  an  investigation  into  the 
phenomena  which  are  associated  with  mass. 

The  conception  of  mass,  in  the  present  sense 
of  the  word,  we  owe  to  Newton  :  before  his  day 
no  clear  distinction  was  made  between  mass  and 


THE  PHILOSOPHICAL  BASIS  27 

weight.  We  cannot  predict  whether  mass,  as 
defined  above,  has  any  relation  to  weight ;  any 
discovery  of  a  connection  between  them  must  be 
a  matter  of  experiment. 

Weight  is  the  force  between  the  earth  and  the 
body  considered,  the  product  of  the  mass  and 
acceleration  being  the  same  for  the  earth  as  for 
the  body.  If  the  forces  were  equal,  the  accele- 
rations towards  the  earth  of  two  bodies  would, 
by  our  definition  of  mass,  be  inversely  propor- 
tional to  their  masses.  By  experiments  on  the 
acceleration,  then,  the  forces  may  be  determined. 
Now  it  was  shown  by  Galileo  that,  if  the  resistance 
of  the  air  be  eliminated,  bodies  fall  at  the  same 
rate  to  the  earth  ;  that  is,  that  the  accelerations 
of  all  bodies  to  the  earth  are  the  same.  It  follows 
that  the  forces,  that  is,  the  weights  of  the  bodies, 
must  be  proportional  to  the  masses.  Masses 
can  thus  be  compared  by  weighing,  and  this 
method  is  much  the  most  convenient  in  practice. 
Nevertheless,  it  must  always  be  remembered  clearly 
that  the  proportionality  between  mass  and  weight, 
and  the  consequent  possibility  of  comparing  masses 
by  means  of  the  balance,  is  not  a  relation  which 
could  be  predicted  a  priori,  but  one  which  has 
been  established  as  the  result  of  careful  experi- 
mental investigation. 

When   we   turn    from  mechanics    to   the    other 


28  PHYSICAL  SCIENCE 

branches  of  physics,  it  is  necessary,  in  the  present 
state  of  knowledge,  to  use  certain  other  funda- 
mental conceptions,  such  as  temperature  and 
quantity  of  electricity,  though  it  is  probable  that 
ultimately  these  will  be  connected  with  the 
mechanical  units.  Again,  in  this  place  it  should 
be  remarked  that  such  a  connection  would  not 
show  that  mechanics  is  necessarily  the  more 
fundamental  science :  it  would  be  quite  as  correct, 
when  the  connection  is  established,  to  express 
mechanical  quantities  in  terms  of  electricity  or 
temperature. 

This  example  leads  us  to  state  in  a  general  form 
the  immediate  object  of  Physical  Science.  The 
physicist  seeks  to  discover  the  relations  between 
different  phenomena,  considered  in  one  limited 
aspect,  and  to  express  those  relations  in  a  definite 
quantitative  way.  Our  minds,  led  by  the  analogy 
with  their  own  volitions,  usually  think  of  one  of  the 
related  phenomena  as  the  cause,  and  of  the  other  as 
the  effect.  The  physical  equation  which  expresses 
the  dependence  of  A  on  B,  or,  in  symbols,  A  =  f(B), 
may  equally  well  be  written  in  the  inverse  form, 
by  which  B  is  asserted  to  be  a  function  of  A.  In 
such  cases,  there  is  probably  no  philosophical 
distinction  between  cause  and  effect ;  it  is  no  more 
right  to  say  that  an  increase  of  pressure  produces 
a  decrease  of  volume  in  a  gas  than  to  say  that  a 


THE  PHILOSOPHICAL  BASIS  29 

decrease  of  volume  produces  an  increase  of  pres- 
sure. The  student  merely  discovers  by  experiment 
that  the  two  phenomena  accompany  each  other 
in  every  case  investigated,  and  sums  up  the  re- 
sults of  experience  in  conceptual  language  and  in 
a  short-hand  form,  in  order  to  save  the  detailed 
investigation  of  each  future  individual  case. 

In  these  examples,  the  needlessness  of  the  ideas 
of  cause  and  effect  will  be  fairly  clear,  as  well  as 
the  confusion  of  thought  sometimes  brought  about 
by  their  introduction.  It  is  where  the  element  of 
time  is  involved  that  the  idea  of  causation  is  most 
vivid.  When  one  of  the  two  related  phenomena 
follows  the  other,  the  mind  instinctively  identifies 
post  hoc  with  propter  hoc.  And,  even  if  such  a  dis- 
tinction is  philosophically  unnecessary,  as  a  matter 
of  convenience  in  language  it  is  perhaps  justified. 
When  carefully  examined,  however,  the  difficulty  of 
isolating  the  "  cause  "  of  any  particular  " effect" 
will  be  found  to  be  insuperable.  A  long  train 
of  circumstances  has  preceded  the  phenomenon 
considered,  and  the  phenomenon  would  not  have 
appeared  had  any  one  of  those  circumstances 
been  absent.  Each  or  all  of  them  might  equally 
well  have  been  called  the  "  cause."  Whether 
the  idea  of  cause  and  effect  represents  a  real 
distinction  in  the  hypothetical  world  which  our 
conceptions  represent,  remains,  like  the  nature 


3o  PHYSICAL  SCIENCE 

and  existence  of  that  world  itself,  an  inquiry  for 
the  philosopher. 

Physical  Science,  then,  seeks  to  establish  general 
rules  which  describe  the  sequence  of  phenomena 
in  all  cases.  Underlying  all  such  attempts  is  the 
belief  that  such  an  orderly  sequence  is  invariably 
present,  could  it  only  be  traced.  This  belief, 
which  is  the  result  of  constant  experience,  is  known 
as  the  principle  of  the  Uniformity  of  Nature.  In 
its  absence  no  organised  knowledge  could  be 
obtained,  and  any  attempt  to  investigate  phenomena 
would  be  perfectly  useless.  Unless,  to  use  the 
conventional  language  justified  above  as  a  matter 
of  convenience,  like  causes  always  produce  like 
effects  in  like  circumstances,  science,  and  indeed 
all  organised  knowledge,  would  be  impossible. 

When  fitted  into  our  mental  picture,  a  generalised 
result  of  experience  is  known  as  a  physical  law, 
or,  to  change  the  form  of  a  word  and  the  size  of 
two  letters,  as  a  Law  of  Nature.  Many  brave 
things  have  been  written,  and  many  capital  letters 
expended  in  describing  the  Reign  of  Law.  The 
laws  of  Nature,  however,  when  the  mode  of  their 
discovery  is  analysed,  are  seen  to  be  merely  the 
most  convenient  way  of  stating  the  results  of 
experience  in  a  form  suitable  for  future  reference. 
The  word  "  law "  used  in  this  connection  has  had 
an  unfortunate  effect.  It  has  imparted  a  kind  of 


THE   PHILOSOPHICAL   BASIS          31 

idea  of  moral  obligation  which  bids  the  phenomena 
"  obey  the  law/'  and  leads  to  the  notion  that,  when 
we  have  traced  a  law,  we  have  discovered  the  ulti- 
mate cause  of  a  series  of  phenomena.  Newton  and 
Ohm  did  not  first  promulgate  and  then  enforce 
the  regulations  which  are  associated  with  their 
names,  though  it  is  not  only  elementary  students 
who  may  be  heard  saying  that  a  stone  falls  to 
the  ground  "  because  of  the  law  of  gravitation." 
We  must  still  ask  why  each  particle  of  one  body 
attracts  each  particle  of  another,  even  when  we 
know  that  the  force  between  them  is  proportional 
to  the  product  of  the  masses  divided  by  the  square 
of  the  distance.  We  do  not  necessarily  know 
why  the  electric  current  through  a  conductor 
varies  as  the  applied  electro-motive  force,  when 
we  have  discovered  how  these  two  quantities  are 
connected. 

The  great  change  in  the  rate  of  progress  of 
Natural  Science  has  occurred  since  men  learned 
to  concentrate  their  immediate  attention  on  the 
question  of  how  phenomena  are  related,  and  to 
cease,  for  the  time  at  any  rate,  to  ask  why  they 
appear.  Before  Galileo's  day  men  sought  to  ex- 
plain the  fall  of  bodies  to  the  earth  by  saying 
that  "every  body  sought  its  natural  place" — the 
place  of  heavy  bodies  being  below,  and  that  of 
light  ones  above.  Galileo,  exercising  the  true 


32  PHYSICAL  SCIENCE 

scientific  spirit  of  restraint,  set  himself  to  de- 
termine by  experiment  how  bodies  fell.  He  thus 
discovered  that  the  speed  was  proportional  to  the 
time  of  fall,  and;  by  dropping  bodies  from  the 
top  of  the  leaning  tower  of  Pisa,  showed  that, 
contrary  to  the  received  doctrine  of  tendency  to 
seek  their  natural  place,  heavy  bodies  fell  no 
faster  than  light  ones. 

The  natural  laws  of  falling  bodies  were  thus 
established,  and  the  method  of  their  discovery  is 
in  truth  an  example  of  the  method  by  which 
such  steps  in  knowledge  are  always  made.  The 
course  of  investigation  is  so  regular  that  the 
process  by  which  a  physical  law  is  discovered 
may  well  be  described  in  general  terms. 

In  the  first  stage  new  phenomena  are  observed, 
or  old  phenomena  are  brought  under  accurate 
and  quantitative  measurement.  Here  the  virtues 
of  patience,  accuracy,  incredulity,  and  conscien- 
tious elimination  of  personal  bias  are  of  chief 
account.  The  classical  example  of  this  process  is 
Kepler's  life-study  of  the  motions  of  the  planets. 

The  data  accumulate ;  they  are  misinterpreted 
and  re-interpreted,  until  some  Newton  with  the 
touch  of  genius  which  often  accompanies  sober 
scientific  insight  and  imagination — some  one  who 
is  able  to  brush  aside  for  a  time  the  non-essential, 
and  to  rise  above  the  confusion  of  detail — is  inspired 


THE  PHILOSOPHICAL  BASIS  33 

with  a  conception  of  order  in  the  multiplicity  of  the 
phenomena  :  order  to  be  seen  when  some  simple 
principle  is  borne  in  mind,  and  is  expressed  in  a 
formula,  which  in  terms  of  our  conceptual  short- 
hand enables  us  to  remember  and  to  predict 
the  sequence  of  phenomena.  If  the  formula  is 
expressed  in  terms  of  simple  conceptions,  already 
known  and  often  used  in  other  branches  of 
knowledge,  the  mind  at  once  looks  on  it  as  an 
"  explanation "  of  the  phenomena,  though  it  is 
evident  on  further  thought  that  the  phenomena 
are  no  more  fully  understood  than  are  the  funda- 
mental conceptions — mass,  force,  whatever  they 
be — in  which  the  "  explanation  "  is  expressed. 

The  next  step  consists  in  deducing  new  conse- 
quences of  the  hypothesis ;  and  here  the  methods 
of  mathematical  analysis  are  usefully  applied.  The 
science  of  mathematics  as  such  has  nothing  to  do 
with  natural  phenomena.  Like  physical  science  it 
is  concerned  with  ideal  conceptions  ;  but  neither 
does  it  seek  to  gain  those  conceptions  from 
an  examination  of  Nature,  nor  to  check  their 
correspondence  by  the  methods  of  experiment. 
Mathematics  may  borrow  subject-matter  from 
observational  science,  or  may  acquire  by  pure 
mental  processes  subject-matter,  such  as  the 
geometry  of  four  dimensional  space,  which  has 
no  counterpart  in  Nature  as  we  know  it.  In 


34  PHYSICAL  SCIENCE 

either  case,  mathematics  deals  with  the  concep- 
tions as  such,  and  traces  their  results  and  the 
relations  between  them  by  the  methods  of  logic, 
with  no  necessary  intention  of  elucidating  the 
phenomena  of  Nature.  In  his  essential  features, 
the  mathematician  is  a  calculating  machine.  His 
conclusions  are,  or  ought  to  be,  contained  im- 
plicitly in  the  premises  he  uses.  He  develops 
the  premises,  discovers  their  full  meaning,  and 
elaborates  their  consequences,  in  a  way  quite 
beyond  the  unaided  power  of  thought,  which, 
without  the  guiding  rules  and  generalisations  of 
mathematical  analysis,  would  be  lost  in  the  maze 
of  complications.  But  the  mathematician  lives  in 
a  purely  conceptual  sphere,  and  mathematics  is 
but  the  higher  development  of  symbolic  logic. 

Taking,  then,  a  new-born  hypothesis,  its  con- 
sequences are  deduced  by  logical  common-sense 
reasoning ;  and,  where  such  reasoning  cannot  see 
its  way  unaided,  by  the  help  of  mathematical 
analysis.  The  results  thus  obtained  are  then 
used  by  the  observer  or  experimenter,  who  tests 
by  the  use  of  old,  or  the  determination  of  new 
data,  the  truth  of  the  formula  by  every  possible 
means.  Its  relations  to  other  ascertained  prin- 
ciples, its  power  of  correlating  hitherto  uncon- 
nected phenomena,  are  examined  in  turn.  From 
consideration  of  its  significance,  we  gain  sug- 


THE  PHILOSOPHICAL  BASIS  35 

gestions  for  further  observation,  if  possible  for 
future  experiment.  Such  experiments,  undertaken 
with  the  express  purpose  in  view,  are  probably 
better  adapted  to  test  the  formula  than  the  blind 
observations  previously  accumulated.  If  the  con- 
cordance is  complete  as  far  as  the  accuracy  of 
experiment  can  go,  the  formula  becomes,  in  the  then 
state  of  knowledge,  a  "  Law  of  Nature."  Whatever 
this  means,  such  a  generalisation  will,  at  all  events, 
prove  a  useful  working  hypothesis,  by  the  light  of 
which  research  may  be  guided  into  promising 
paths.  As  the  range  of  observation  widens,  and 
as  the  accuracy  of  the  old  observations  is  in- 
creased, the  fate  of  the  new  law  hangs  in  the 
balance.  The  formula  may,  perhaps,  still  be 
confirmed,  it  may  require  modification,  or  it  may 
have  to  be  abandoned  as  a  theory  which  has 
played  a  useful  and  honourable  part  in  its  day, 
but  has  become  inadequate  to  express  the  de- 
veloping knowledge  of  a  later  time.  If  so,  it 
ceases  to  be  cited  as  a  "  Law  of  Nature."  Not 
that  Nature  has  changed,  but  rather  our  attitude 
towards  her,  and  our  conceptual  model  of  her 
phenomena.  Thus  new  laws  replace  the  old  ones. 
Some  years  ago  the  constancy  of  the  chemical 
elements  was,  in  the  then  state  of  knowledge,  a  law 
of  Nature.  Latterly,  the  phenomena  of  radio-activity 
have  forced  us  to  believe  that  radium  is  passing 


36  PHYSICAL  SCIENCE 

continuously  and  spontaneously  into  helium — that 
true  transmutations  of  matter  occur.  The  obvious 
transmutation  of  one  kind  of  matter  leads  to  the 
possibility,  nay,  the  probability,  of  the  gradual 
transmutation  of  all ;  since  as  yet  no  property  of 
matter  has  been  noted  which  is  the  exclusive 
possession  of  one  substance  alone.  New  pheno- 
mena, or  rather  phenomena  for  the  first  time 
appreciated,  are  continually  coming  to  light,  and 
evidence  is  accumulating  from  which  the  pro- 
fitable construction  of  theories  —  for  a  time  in 
abeyance — may  again  be  pursued.  Nothing  must 
be  ruled  out  of  court  because  contrary  to  re- 
ceived views ;  when  a  primd  facie  case  has  been 
made  out,  everything  must  be  examined  by  ex- 
periment, induction,  deduction,  and  again  experi- 
ment. This  is  the  only  sure  road  to  the  under- 
standing of  Nature ;  and,  in  times  to  come,  it  may 
lead  us  into  regions  now  unknown,  or  considered 
to  be  closed  to  the  investigations  of  science. 
The  evolution  and  disintegration  of  matter,  the 
problems  of  hypnotism  and  of  direct  thought 
transference,  are  questions  which  seem  to  be 
rapidly  coming  within  the  range  of  scientific 
inquiry.  It  is  possible  that  an  advance  has 
already  been  made  towards  clearing  away  part 
of  the  mystery,  so  attractive  to  some,  so  repellent 
to  others,  that  surrounds  these  phenomena.  At 


THE   PHILOSOPHICAL  BASIS  37 

any  rate,  in  several  of  the  great  schools  of 
psycho-medicine,  notably  in  France  and  America, 
materials  are  being  accumulated,  their  trust- 
worthiness examined,  and  the  results  systemati- 
cally collated.  It  may  be  that  these  investigations, 
so  beset  with  evident  difficulties,  are  indeed  in- 
definitely complicated  in  their  issues  by  questions 
of  racial  predisposition,  of  individual  temperament 
and  mental  condition.  Whether  any  or  all  of  these 
problems  \vill  prove  amenable  to  the  methods  of 
dispassionate  observation  and  experiment  is  a 
matter  which  the  years  to  come  alone  can  show. 
We  must  thus  look  on  natural  laws  merely  as 
convenient  shorthand  statements  of  the  organised 
information  that  is  at  present  at  our  disposal. 
But  when  Physical  Law,  as  understood  in  the 
eighteenth  century,  has  been  dethroned  from  a 
place  that  was  never  rightly  its  own,  let  us  not 
think  that  its  usefulness  has  been  diminished  or 
its  dignity  unduly  lowered.  Without  the  possi- 
bility of  framing  such  laws,  mankind  would  be 
hopelessly  lost  in  a  wilderness  of  phenomena  ;  no 
continuous  progress  could  be  made ;  no  consistent 
idea  of  the  world  around  could  ever  be  attained. 
Each  individual  phenomenon,  as  it  appeared  time 
after  time,  might  still  be  investigated  ;  but,  with 
his  limited  mind  and  short  life,  no  one  man 
could  ever  secure  a  basis  for  adequate  know- 


38  PHYSICAL  SCIENCE 

ledge.  Without  some  general  way  of  stating  his 
experiences,  he  could  hand  on  neither  his  guesses 
after  truth  nor  his  hard-won  information  :  mankind 
would  never  have  emerged  from  barbarism. 

The  fundamental  conceptions  of  length,  time,  and 
mass  from  which,  as  we  have  seen,  the  other 
mechanical  units  can  be  derived,  enable  us  to  con- 
struct a  mechanical  model  of  Nature.  It  is  incom- 
plete ;  for  even  the  simplest  mechanical  fact,  such  as 
the  fall  of  a  body  to  the  ground,  inevitably  has  other 
aspects.  Heat  may  be  developed,  electrical  mani- 
festations appear,  and,  if  the  body  be  a  living  one, 
physiological  and  psychological  changes  take  place. 
Neglecting  these  aspects,  however,  a  complete 
mechanical  account  of  the  phenomenon  can  be 
given  in  terms  of  the  three  fundamental  concep- 
tions. As  we  have  seen,  new  ideas,  which  may  be 
derived  from  the  primary  ones,  become  necessary 
in  the  course  of  the  investigation.  The  body  falls 
with  a  certain  acceleration,  and,  at  any  instant,  is 
moving  with  a  definite  velocity.  As  it  falls,  it 
acquires  energy  of  motion  and  loses  energy  of  posi- 
tion. 

During  the  fall  we  find  that  we  can  successfully 
describe  what  happens  by  assuming  that  the 
quantity  which  we  call  the  mass  of  the  body  keeps 
constant,  and  that  the  sum  of  the  two  kinds  of 


THE  PHILOSOPHICAL  BASIS  39 

energy  keeps  constant  also.  If  we  include  in  our 
view  the  complete  physical  and  chemical  aspects  of 
the  phenomena,  we  may  greatly  extend  these  results. 
When  the  body  reaches  the  earth,  it  is  possible  that 
processes  of  decay  set  in,  which  eventually  result  in 
most  of  its  substance  disappearing  in  gases  or  other 
products.  The  energy  of  motion  acquired  by  the 
body  during  its  fall  also  seems  to  disappear,  with 
no  corresponding  gain  of  energy  of  position. 
Chemistry,  however,  generalising  from  many  ex- 
perimental results,  tells  us  that,  if  we  could  trace  all 
the  forms  of  matter  into  which  the  body  is  resolved, 
we  should  find  that  there  was  no  loss.  Every 
particle  of  the  original  body  still  exists  in  one  of  its 
products.  Physics,  on  the  other  hand,  teaches  us 
in  the  same  way  that  the  sum  of  all  the  forms  of 
energy,  heat,  sound,  &c.,  which  appear  as  a  con- 
sequence of  the  impact  on  the  ground,  could  they 
all  be  taken  into  account,  would  be  exactly  equi- 
valent to  the  energy  of  motion  possessed  by  the 
body  at  the  instant  before  contact.  These  great 
principles  of  the  conservation  of  mass  and  the 
conservation  of  energy  are  two  of  the  most  im- 
portant generalisations  ever  reached  by  Physical 
Science. 

While  fully  recognising  the  importance  of  these 
generalisations  from  the  physical  point  of  view,  we 
must  be  careful  how  we  give  them  any  metaphysical 


40  PHYSICAL  SCIENCE 

significance.  Under  certain  limiting  conditions, 
other  physical  quantities  besides  mass  and  energy 
may  be  conserved.  Thus  in  pure  mechanics  the 
conservation  of  momentum  is  well  recognised,  while 
in  reversible  systems  thermodynamics  indicates 
the  conservation  of  another  quantity,  named  by 
Clausius,  entropy.  Momentum  and  entropy  are 
only  conserved  under  restricted  conditions ;  in 
physical  systems  the  momentum  of  visible  masses 
is  often  destroyed,  while  in  irreversible  processes 
entropy  always  tends  to  increase. 

Mass  and  energy  may  seem  to  be  conserved 
in  the  conditions  known  to  us,  and  we  are 
justified  in  extending  the  principle  of  their  con- 
servation to  all  cases  where  those  conditions  apply. 
It  does  not  follow,  however,  that  conditions  un- 
known to  us  may  not  exist,  in  which  mass  and 
energy  might  disappear  or  come  into  existence. 
The  persistence  of  matter,  for  instance,  might  con- 
ceivably be  an  apparent  persistence.  A  wave,  travel- 
ling over  the  surface  of  the  sea,  seems  to  persist.  It 
keeps  its  form  unchanged,  and  the  quantity  of  water 
in  it  remains  unaltered.  We  might  talk  about  the 
conservation  of  waves,  and,  perhaps,  in  so  doing,  be 
as  near  the  truth  as  when  we  talk  of  the  persistence 
of  the  ultimate  particles  of  matter.  But  the  persis- 
tence of  the  wave  is  an  apparent  phenomenon.  The 
form  of  the  wave  indeed  truly  persists,  but  the 


THE   PHILOSOPHICAL  BASIS  41 

matter  in  it  is  always  changing — changing  in  such 
a  way  that  successive  portions  of  matter  take,  one 
after  the  other,  an  identical  form.  Indications  are 
not  wanting  that  only  in  some  such  sense  as  this  is 
mass  persistent.  The  conservation  of  mass  and 
energy  under  all  known  conditions  is  a  valid  meta- 
physical argument  in  favour  of  the  view  that  our 
ideas  of  them  correspond  with  ultimate  realities,  but 
it  is  no  more  than  an  argument ;  it  deserves  due 
weight,  but  it  is  not  conclusive  evidence. 

Even  if  we  assume  that  some  reality  underlies 
phenomena,  it  is  clear  that  the  reality  must  be  very 
different  from  the  mental  picture  which  common- 
sense  frames,  when  unaided  by  the  inductions  of 
science.  Our  first  conception  of  a  wooden  stick 
involves  the  ideas  of  a  certain  long-shaped  form,  of 
hardness,  of  weight,  of  a  colour  more  or  less  brown, 
perhaps  of  some  amount  of  elasticity.  Examination 
with  a  microscope  reveals  many  appearances  in- 
visible with  the  unaided  eye,  and  we  find  that  the 
stick  has  a  structure  much  more  detailed  than  we 
imagined.  From  the  results  of  observation  and 
experiment,  physics  teaches  us  that  the  properties 
of  the  stick  can  only  satisfactorily  be  represented  by 
the  hypothesis  that  the  substance  of  it  is  divisible, 
but  not  infinitely  divisible;  that  it  consists  of  discon- 
tinuous particles  or  molecules.  Again,  chemistry 
assures  us  that  the  molecules  of  the  stick  are 


42  PHYSICAL  SCIENCE 

made  up  of  still  smaller  parts  or  atoms,  which 
separate  from  each  other  when  chemical  action 
occurs,  when,  for  instance,  the  stick  is  burnt, 
and  can  afterwards  re-arrange  themselves  into 
new  molecules. 

When  we  pursue  our  inquiries  into  the  nature  of 
these  chemical  atoms,  we  find  that  recent  research 
has  resolved  them,  as  we  shall  see  later,  into  much 
smaller  particles  or  corpuscles,  and  we  are  asked 
to  imagine  that  these  are  in  constant  motion  within 
the  atom,  somewhat  as  the  planets  move  within 
the  solar  system.  Intimate  relations  exist  between 
these  corpuscles  and  the  phenomena  of  electricity, 
and  it  seems  probable  that  a  corpuscle  may  be 
regarded  as  an  isolated  electric  charge,  or  electron, 
as  it  is  called,  the  mass  of  the  corpuscle  being 
an  apparent  effect  due  to  electricity  in  motion. 

Thus  we  have  "  explained "  electricity  in 
terms  of  corpuscles,  and  mass  itself  in  terms  of 
electricity.  At  present  adventurous  pioneers  are 
striving  to  escape  from  the  circle  and  to  reach 
more  ultimate  conceptions  by  resolving  the  cor- 
puscle or  electron  into  a  centre  of  intrinsic  strain 
in  the  luminiferous  aether.  Whatever  fate  may 
await  their  efforts,  we  have  already  travelled  far 
in  attempting  to  construct  a  complete  mental 
image  of  the  wooden  stick  and  all  its  known 
properties.  We  have  reached  ideas  very  different 


THE  PHILOSOPHICAL  BASIS          43 

from  those  of  the  hard,  continuous  substance  from 
which  we  started. 

The  other  properties  of  the  stick  can  be  analysed 
into  physical  conceptions  in  much  the  same  way. 
Thus  the  colour  is  found  to  be  due  to  a  sorting 
action  which  the  particles  of  the  wood  exert  on  the 
complex  system  of  aethereal  waves,  making  up  white 
light.  Some  of  these  waves  have  their  energy  more 
freely  absorbed  by  the  molecules  of  the  wood  than 
have  others ;  the  balance  of  light  is  upset,  and  the 
reflected  beam  produces  the  sensation  of  colour. 
Here,  again,  the  most  fundamental  conceptions 
into  which  modern  science  enables  us  to  resolve 
our  primitive  ideas  are  very  different  from  those 
in  which  they  took  their  origin. 

While  Natural  Science  is  not  committed  to  any 
particular  philosophical  system,  while  in  its  essence 
it  is  independent  of  all  such  systems,  the  language 
it  uses  habitually  is  based  on  the  common-sense 
realism,  which  is  the  philosophic  creed  of  most 
men  of  science — indeed,  of  the  great  bulk  of  man- 
kind, or  at  all  events,  of  that  part  of  mankind 
belonging  to  the  races  of  Western  Europe.  The 
mass  and  energy  with  which  we  deal  in  physical 
experiments,  and  in  the  mathematical  reasoning 
based  on  inductions  from  the  experiments,  are 
purely  conceptual  quantities,  introduced  to  bring 


44  PHYSICAL  SCIENCE 

order  and  simplicity  into  our  perceptions  of  pheno- 
mena. But  science  talks  of  matter  and  energy  as 
though  it  knew  of  the  existence  of  realities  corre- 
sponding to  the  mental  images  to  which  alone  these 
names  strictly  apply.  In  the  laboratory,  as  in 
practical  life,  there  is  neither  room  nor  time  for 
philosophic  doubt  In  periods  of  reflection,  how- 
ever, when  considering  the  theoretical  bearing  of 
the  results  of  our  experiments,  it  is  sometimes 
well  to  remember  the  limitation  of  our  present 
certain  knowledge,  and  the  purely  conceptual 
nature  of  our  scheme  of  Natural  Science  when 
based  merely  on  its  own  inductions. 


CHAPTER    II 

THE    LIQUEFACTION    OF    GASES    AND    THE 
ABSOLUTE    ZERO    OF    TEMPERATURE 

"Scientia  et  potentia  humana  in  idem  coincidunt,  quia  ignoratio 
causae  destituit  effectum." — BACON,  Novum  Organum. 

MATTER  is  known  to  us  in  three  states — as  solid, 
as  liquid,  and  as  gas.  The  relations  between  these 
three  states  have  been  the  subject  of  investigation 
throughout  the  history  of  Physical  Science,  and, 
indeed,  almost  throughout  the  history  of  the  human 
race.  The  solidification  of  water  in  a  frost,  and 
its  evaporation  by  the  sun  or  a  fire,  have  been 
familiar  to  mankind  from  the  earliest  times.  But 
water  shows  these  changes  of  state  under  too 
favourable  an  aspect  to  be  taken  as  a  general 
example.  It  has  by  no  means  always  been  clear 
that  such  transformations  were  possible  to  all 
kinds  of  matter,  and  it  has  been  necessary  to 
exhaust  the  resources  of  modern  civilisation  to 
liquefy  the  more  permanent  gases. 

Ice,  when  heat  is  supplied,  begins  to  melt  at  a 
definite   temperature,    which    is   called    o°   on   the 


46  PHYSICAL  SCIENCE 

Centigrade  scale,  and  32°  on  the  scale  devised  by 
Fahrenheit.  While  any  ice  remains,  no  change 
of  temperature  occurs  in  the  mixture  of  ice  and 
water.  Heat  is  still  absorbed,  but  its  energy  is 
used  to  effect  a  change  of  state,  not  to  raise  the 
temperature.  The  pure  substance  ice  has  a  con- 
stant melting-point.  Similarly,  if  water  be  cooled 
at  constant  pressure,  it  begins  and  finishes  to  freeze 
at  the  same  temperature.  It  has  a  constant  freez- 
ing-point, identical  with  the  melting-point. 

When  water  boils,  a  still  larger  quantity  of  heat 
is  absorbed,  but  the  temperature  again  remains 
unaltered  during  the  whole  process.  When  the 
barometer  stands  at  760  millimetres,  or  just  under 
30  inches,  of  mercury,  the  temperature  of  the  boil- 
ing-point is  taken  as  the  second  fixed  point  on  our 
thermometers,  and  called  100°  or  212°  according 
as  we  use  the  Centigrade  or  the  Fahrenheit  scale. 
If  the  barometer  stands  higher  or  lower  than  the 
standard  height,  the  boiling-point  of  water  is  found 
to  be  above  or  below  100°  C.,  rising  or  falling 
through  i°  C.  for  a  change  of  27  millimetres  in 
the  barometer.  The  freezing-point  also  depends 
on  the  pressure ;  but  the  change  is  much  smaller 
than  in  the  case  of  the  boiling-point,  and  delicate 
experiments  are  necessary  to  determine  it. 

The  variation  with  pressure  of  the  points  of 
transition  from  one  state  of  matter  to  another  are 


THE   LIQUEFACTION  OF  GASES       47 

connected  with  the  changes  of  volume  which  simul- 
taneously occur.  Water  expands  on  freezing,  for 
ice  floats  on  the  surface  of  a  lake,  and  pipes  burst 
in  a  frost.  If  this  increase  in  volume  be  resisted 
by  an  external  pressure,  as  by  putting  the  water 
into  a  strong  closed  vessel,  the  act  of  freezing 
involves  the  performance  of  external  work  in 
forcing  outwards  the  walls  of  the  vessel  to  give 
room  for  the  ice  to  form.  It  is  therefore  more 
difficult  to  produce  ice  under  pressure,  and  a 
greater  lowering  of  temperature  is  necessary.  Thus 
an  increase  of  pressure  must  lower  the  melting  or 
freezing-point.  On  evaporation,  the  increase  in 
volume  occurs  with  the  change  from  liquid  to 
vapour ;  an  increase  of  external  pressure  there- 
fore makes  evaporation  more  difficult,  and  con- 
sequently produces  a  rise  in  the  boiling-point.  If 
the  change  in  volume  and  the  amount  of  heat 
required  to  produce  the  change  in  state  are  known, 
the  principles  of  thermodynamics  enable  us  to 
calculate  the  exact  amount  of  alteration  in  the 
freezing  or  boiling-points. 

There  is  reason  to  suppose  that  the  three  states 
of  solid,  liquid,  and  gas,  assumed  within  a  mode- 
rate range  of  temperature  and  pressure  by  the 
familiar  substance  water,  could  be  obtained  with 
all  bodies  if  we  could  command  temperatures  and 


48  PHYSICAL  SCIENCE 

pressures  high  enough  and  low  enough.  Metals 
melt  and  volatilise  at  high  temperatures,  while 
even  such  gases  as  air  and  hydrogen  have  re- 
cently been  liquefied. 

Several  gases,  previously  unknown  in  any  other 
form,  were  liquefied  by  Faraday.  His  method 
consisted  in  evolving  the  gas  by  heating  chemical 
re-agents  in  one  limb  of  a  bent  glass  tube,  and 
cooling  the  other  limb  in  cold  water  or  a  freezing 
mixture.  As  the  gas  is  evolved,  the  pressure  rises, 
and  either  the  gas  is  liquefied  in  the  cold  limb,  or 
the  tube  bursts.  By  this  simple  means  chlorine, 
sulphur  dioxide,  ammonia,  and  a  few  other  gases 
may  be  liquefied. 

The  conditions  necessary  for  liquefaction  were 
not  fully  understood  till  Andrews,  in  1863,  showed 
that  carbonic  acid  gas  could  not  be  liquefied  unless 
its  temperature  was  reduced  below  a  definite  fixed 
point,  which  he  called  the  critical  point.  The 
critical  point  of  carbonic  acid  is  fairly  high,  about 
30°  on  the  Centigrade  scale  ;  but  for  other  gases, 
such  as  air  or  hydrogen,  it  is  much  lower,  many 
degrees  below  the  freezing-point  of  water.  How- 
ever low  it  be,  unless  a  gas  is  cooled  to  its  critical 
point,  no  pressure,  whatever  be  its  intensity,  can 
produce  liquefaction.  Below  their  critical  points, 
gases  may  be  considered  as  vapours,  and  will 
liquefy  if  the  pressure  applied  is  high  enough. 


THE  LIQUEFACTION   OF  GASES         49 

The  problem  of  the  liquefaction  of  a  refractory 
gas  is  thus  solved  if  we  can  produce  cold  suffi- 
ciently intense  to  reduce  it  below  its  critical  point. 
Three  methods  have  been  used,  either  singly  or 
in  conjunction,  to  cool  gases  below  their  critical 
points.  The  first  method  depends  on  the  heat 
which  it  is  necessary  to  supply  in  order  to  evaporate 
a  liquid.  A  liquid  boils  when  the  pressure  of  its 
vapour  is  equal  to  the  pressure  of  the  atmosphere 
acting  upon  its  surface,  and,  if  we  reduce  this 
external  pressure,  the  boiling-point  is  lowered. 
Thus,  by  pumping  away  the  vapour  as  fast  as  it 
is  formed,  and  so  keeping  the  pressure  low,  a 
liquid  can  be  boiled  at  a  temperature  much  below 
its  normal  boiling-point.  By  this  method,  for 
example,  it  is  possible  to  make  water  boil  with 
no  outside  supply  of  heat.  The  heat  necessary 
for  evaporation  is  then  taken  from  the  water  itself, 
which  in  this  way  is  gradually  cooled.  If  the  air- 
pump  is  efficient,  and  if  very  little  heat  is  allowed 
to  leak  in,  the  cooling  may  go  so  far  that  the  re- 
maining water  is  frozen.  Beginning  at  the  normal 
boiling-point  of  water,  we  should  then  have  cooled 
the  system  by  means  of  evaporation  through  100°. 
If,  instead  of  water,  we  had  taken  some  liquid  of 
low  boiling-point,  such  as  liquefied  sulphur  dioxide, 
or,  better  still,  liquefied  carbonic  acid,  the  same 
process  of  cooling  under  exhaustion  would  have 


50  PHYSICAL  SCIENCE 

taken  place ;  but  the  final  temperature  reached 
would  have  been  much  lower. 

Starting  then  with  some  substance  like  liquefied 
sulphur  dioxide,  which  is  easily  liquefied  by  pressure 
alone  at  ordinary  temperatures,  we  can  boil  it  away 
under  exhaustion,  and  so  produce  a  low  tempera- 
ture. By  making  a  more  refractory  gas,  such  as 
carbonic  acid,  circulate  through  a  tube  surrounded 
with  the  cooled  sulphur  dioxide,  this  new  agent  is 
cooled  below  its  critical  point,  and  liquefied.  In  its 
turn  the  liquid  carbonic  acid  is  boiled  away  under 
low  pressure,  and  used  as  a  refrigerating  agent  to 
cool  the  gas — oxygen,  let  us  say — which  we  are 
attempting  to  conquer.  This,  sometimes  called  the 
cascade  method  of  cooling,  was  the  plan  adopted 
by  the  Swiss  physicist,  Pictet  of  Geneva,  in  the 
experiments  which,  simultaneously  with  those  of 
his  French  contemporary  Cailletet,  first  liquefied 
oxygen.  With  one  of  those  curious  coincidences 
which  the  broad  wave  of  advancing  knowledge 
sometimes  produces,  both  these  results  were  an- 
nounced at  a  memorable  meeting  of  the  French 
Academy,  held  on  the  24th  of  December  1877. 

Even  when  the  gas  was  thus  cooled,  however, 
Pictet's  process  was  not  entirely  effective.  In  order 
to  pass  the  last  few  degrees  and  reach  the  critical 
point,  a  second  method  of  cooling  had  to  be 
brought  into  play.  To  explain  this  second  method 


THE  LIQUEFACTION  OF  GASES         51 

other  principles  must  be  taken  into  account.  When 
a  certain  mass  of  gas,  forced  into  a  closed  vessel 
till  the  pressure  rises  to  several  atmospheres,  is 
suddenly  let  out,  its  volume  is,  of  course,  greatly 
increased  by  the  sudden  expansion.  Room  has  to 
be  made  for  the  increase  of  volume,  and  to  do  so  re- 
quires the  expenditure  of  work,  for  the  atmosphere  is 
pressing  on  the  gas  on  all  sides,  and  has  to  be  forced 
back  when  the  expansion  occurs.  Moreover,  if  the 
particles  of  the  gas  attract  each  other,  work  must 
be  done  in  the  separation  necessary  for  the  increase 
of  volume.  Thus  internal  as  well  as  external  work 
may  be  performed  during  the  expansion.  Unless 
heat  is  supplied  from  without,  the  energy  needed 
to  perform  all  this  work  must  come  from  the  heat 
supply  of  the  gas  itself,  which  becomes  cooled 
in  the  'process.  If  the  expansion  is  sudden  and 
therefore  rapid,  there  is  no  time  for  heat  to  enter 
the  gas,  and  the  cooling  represents  the  full  effect 
of  the  work  done.  By  this  means,  Pictet  finally 
liquefied  his  oxygen.  The  highly  compressed  gas, 
which  had  been  cooled  in  liquid  carbonic  acid 
boiling  under  low  pressure,  was  allowed  suddenly 
to  escape  into  the  atmosphere.  A  large  amount 
of  external  work  was  thus  done,  intense  cooling 
resulted,  and  liquid  oxygen  was  seen  as  spray  in 
the  issuing  jet  of  gas.  It  was  by  a  still  more 
sudden  expansion  that  Cailletet  liquefied  oxygen, 


52  PHYSICAL  SCIENCE 

using  only  preliminary  cooling  to  30°  below  the 
Centigrade  zero. 

In  modern  forms  of  apparatus  for  the  liquefaction 
of  gases  it  is  found  advisable  to  sacrifice  the  cooling 
gained  by  the  performance  of  external  work,  and 
to  rely  on  that  due  to  the  internal  work  alone.  By 
this  means  it  is  possible  to  construct  much  more 
powerful  and  efficient  refrigerating  machines.  The 
essential  feature  in  the  process  of  cooling  by  the 
performance  of  external  work  is  the  expansion  of 
the  gas  by  its  own  elastic  force.  If  the  work  neces- 
sary for  the  increase  of  volume  under  the  external 
pressure  be  supplied  by  an  engine,  or  if  all  such 
work  be  prevented  by  making  the  gas  expand  into  a 
vacuum,  there  is  no  external  work  to  absorb  the  heat 
energy  of  the  gas  itself,  and  no  cooling  from  this 
cause  is  produced.  The  gas,  however,  still  has  to 
supply  any  work  needed  to  separate  its  own  particles 
against  any  mutual  attractive  forces,  and,  if  such 
forces  exist,  cooling  can  still  be  obtained  at  the 
expense  of  the  heat-energy  of  the  gas.  On  the 
other  hand,  if  the  inter-molecular  forces  are  forces 
of  repulsion,  expansion  will  be  aided  by  their  action, 
and  will,  in  the  absence  of  external  work,  be  ac- 
companied by  an  increase  of  temperature.  Thus, 
by  arranging  for  free  expansion,  as  it  is  called,  we 
can  examine  the  nature  of  the  inter-molecular  forces 
by  observing  whether  a  gas  is  cooled  or  heated. 


THE   LIQUEFACTION   OF  GASES         53 

In  such  experiments,  it  is  necessary  to  prevent 
the  performance  of  external  work  by  the  gas  itself, 
and  this  can  be  done  in  either  of  the  two  ways 
indicated  above.  Gay  Lussac,  and  afterwards 
Joule,  filled  one  vessel  with  gas  under  high  pres- 
sure, and  then  allowed  it  to  expand  into  another 
vessel  previously  exhausted.  Here,  in  expanding 
into  a  vacuum,  no  external  pressure  has  to  be  over- 
come, and  no  external  work  is  done.  Any  thermal 
change  will  be  the  equivalent  of  the  internal  work. 
The  vessels  were  placed  side  by  side  in  water, 
which  was  stirred  after  the  experiment,  and  tested 
with  a  sensitive  thermometer.  At  ordinary  tem- 
peratures no  heating  or  cooling  could  be  observed 
with  any  of  the  gases  examined. 

The  apparatus  just  described  is  clearly  not 
adapted  to  detect  small  thermal  changes,  and  it 
was  not  till  about  the  year  1850,  when  Thomson  and 
Joule  devised  a  continuous  method,  that  satisfactory 
results  were  obtained.  Instead  of  preventing  ex- 
ternal work  by  allowing  the  gas  to  expand  into 
a  vacuum,  these  physicists  performed  the  external 
work  needed  to  expand  the  gas  against  the  pressure 
of  the  atmosphere  by  means  of  an  air-pump  driven 
by  an  engine.  By  this  method  a  continuous 
current  of  gas  was  forced  through  a  porous  plug  of 
compressed  wool  or  silk,  fixed  in  a  wooden  tube. 
Here  the  engine  does  the  external  work,  and  con- 


54  PHYSICAL  SCIENCE 

sequently  none  of  that  work  draws  on  the  heat 
energy  of  the  gas  itself. 

All  the  external  work  is  done  by  the  engine,  but, 
as  we  have  seen,  another  source  of  energy-changes 
exists.  When  a  gas  expands,  whether  or  not  it 
performs  external  work,  the  various  parts  of  it 
become  separated  further  from  each  other,  since, 
on  the  whole,  the  gas  occupies  after  expansion  a 
larger  volume  than  before.  If,  then,  there  is  any 
attraction  between  the  parts  of  the  gas,  work  must 
be  done  in  separating  them  ;  in  terms  of  the 
molecular  theory,  work  is  done  against  the  inter- 
molecular  forces.  For  the  performance  of  this 
internal  work,  energy  must  be  drawn  from  the 
heat-supply  of  the  gas,  which  will  therefore  cool, 
and  the  amount  of  cooling,  if  access  of  heat  from 
outside  be  prevented,  measures  the  intensity  of 
the  inter-molecular  forces.  On  the  other  hand,  if 
the  inter-molecular  forces  be  repulsive  ones,  they 
help  on  the  expansion,  and  the  energy  so  liberated 
appears  as  sensible  heat,  the  resultant  rise  of 
temperature  depending  on  the  strength  of  the 
repulsion  between  the  molecules. 

The  porous  plug  experiment  was  originally  de- 
vised by  Professor  William  Thomson,  now  Lord 
Kelvin,  and  the  late  Dr.  Joule,  for  the  purpose 
of  examining  the  amount  and  nature  of  these  inter- 
molecular  forces,  and  of  determining  the  amount 


THE  LIQUEFACTION  OF  GASES         55 

of  deviation  of  various  gases  from  the  ideal  state, 
in  which  no  such  forces  exist.  If  a  thermometer 
were  filled  with  such  a  hypothetical  ideal  gas,  its 
indications  would  exactly  coincide  with  the  absolute 
temperature  scale,  deduced  by  Thomson  from  the 
principles  of  thermodynamics.  The  knowledge  of 
the  deviation  of  any  real  gas  from  the  ideal  state 
thus  enables  the  absolute  scale  to  be  compared  with 
the  scale  of  an  actual  thermometer,  using  the  ex- 
pansion of  the  gas  in  question  as  the  thermometric 
property.  The  great  theoretical  importance  of 
the  porous  plug  experiment  will  now  be  manifest. 

Thomson  and  Joule  found  that  air,  and  all  other 
gases  except  hydrogen,  were  slightly  cooled  on 
passing  the  plug ;  with  hydrogen,  on  the  other 
hand,  they  obtained  a  still  smaller  heating  effect. 
Thus  in  hydrogen  the  molecules  must  on  the 
whole  repel  each  other,  while  in  air  and  similar 
gases,  the  intermolecular  forces  must  be  attrac- 
tive ones.  The  amount  of  the  effect  was  found 
to  increase  in  proportion  to  the  difference  of 
pressure  on  the  opposite  sides  of  the  plug. 

With  air  the  cooling  effect  decreases  as  the 
temperature  is  raised,  and  increases  if  the  air 
be  cooled.  The  change  of  temperature  pro- 
duced, which  was  only  one-fifth  of  a  degree 
per  atmosphere  difference  of  pressure  in  the 
original  experiments,  can  thus  be  increased 


56  PHYSICAL  SCIENCE 

to  any  extent  by  a  preliminary  cooling  of 
the  air. 

This  cooling  by  the  performance  of  internal 
work  underlies  the  third  method  adopted  in  the 
liquefaction  of  gases.  It  must  be  distinguished 
clearly  from  the  second  method,  in  which 
most  of  the  cooling  is  effected  by  making  the 
gas  do  external  work. 

Let  us  imagine  that  a  stream  of  air,  previously 
cooled  by  liquid  carbonic  acid,  is  forced  through  a 
spiral  tube  by  aid  of  an  air-pump  and  engine,  and 
that  finally  it  emerges  through  a  fine  nozzle  at  the 
end  of  the  tube.  The  nozzle  acts  as  a  porous  plug, 
and  the  air,  cooled  by  free  expansion,  is  lowered  in 
temperature  by  doing  internal  work.  Let  us  further 
suppose  that  the  issuing  air,  so  cooled,  is  made 
to  flow  back  over  the  tube  through  which  the 
stream  of  air  passes.  The  advancing  current  of  air 
is  still  further  cooled,  the  effect  of  the  expansion 
at  the  nozzle  is  increased,  and  a  temperature  yet 
lower  than  before  attained.  This  cycle  of  opera- 
tions— the  continual  passage  of  the  air  just  cooled 
by  free  expansion  over  the  current  of  air  before 
it  issues  from  the  nozzle  —  results  in  a  con- 
stantly decreasing  temperature,  and  eventually 
cools  the  air  below  its  critical  point,  finally 
causing  liquefaction.  This  self -intensifying  action 
is  sometimes  referred  to  as  the  regenerative 


THE  LIQUEFACTION  OF  GASES         57 

principle.  It  was  first  applied  successfully  to  the 
liquefaction  of  air  by  Linde  in  Germany,  and  has 
since  been  used  by  Hampson  and  Dewar  in  Eng- 
land, and  by  Tripler  in  America. 

Liquid  air  can  be  obtained  in  any  quantity  by 
the  expenditure  of  power,  and  the  necessary  appa- 
ratus has  become  part  of  the  usual  equipment  of 
physical  and  chemical  laboratories.  By  this  means 
regions  of  temperature  before  quite  inaccessible 
have  been  opened  up  to  investigation,  and  the 
use  of  liquid  air  promises  to  be  of  increasing  ad- 
vantage in  many  departments  of  research.  It 
would,  of  course,  be  possible  to  drive  an  engine 
by  means  of  liquid  air,  but  such  a  process  would 
be  very  uneconomical.  The  statements,  which  have 
sometimes  appeared  in  the  daily  papers,  announc- 
ing impending  revolutions  in  methods  of  obtaining 
cheap  power  by  the  application  of  liquid  air,  have 
originated  from  an  imperfect  comprehension  of  the 
problems  involved. 

When  air  had  been  successfully  liquefied,  hydro- 
gen was  obviously  the  next  gas  to  be  attacked. 
Thomson  and  Joule's  porous  plug  experiments  had 
shown  that,  at  ordinary  temperatures,  hydrogen 
suffers  a  heating  effect  on  free  expansion.  It  was 
therefore  useless  to  attempt  to  liquefy  it  by  re- 
generative cooling  alone.  But,  just  as  the  cool- 
ing effect  in  the  case  of  air  increases  as  the  air 


58  PHYSICAL  SCIENCE 

is  subjected  to  a  preliminary  cooling,  so  in 
hydrogen,  if  it  be  first  cooled,  the  Thomson- 
Joule  heating  effect  first  diminishes  and  then 
is  reversed,  becoming  a  cooling  effect.  This 
reversal  was  shown  by  Olszewski  to  take  place 
about  80°  below  the  Centigrade  zero.  Dewar  then 
subjected  hydrogen  to  a  preliminary  cooling  in 
liquid  air  boiling  in  a  vacuum  at  a  temperature  of 
—  205°,  and  afterwards  forced  the  hydrogen  through 
a  regenerative  coil  under  a  pressure  of  180 
atmospheres. 

By  this  means  liquid  hydrogen  was  first  collected 
in  an  open  vessel  on  May  10,  1898,  though  two 
years  before  it  had  been  seen  as  spray  in  the  jet  of 
gas  issuing  from  a  simpler  apparatus  of  the  same 
essential  form.  When  about  20  cubic  centimetres 
of  liquid  had  been  collected  the  later  experiment 
failed,  owing  to  the  stoppage  of  the  exit  by  frozen 
air,  a  very  common  accident  in  dealing  with  liquid 
hydrogen. 

By  working  with  carefully  purified  gas,  much 
larger  volumes  have  since  been  obtained,  and 
the  writer  has  a  vivid  memory  of  an  afternoon 
in  June  1901,  when  Professor  Dewar  had  trans- 
ported some  five  litres  of  liquid  hydrogen  from  the 
Royal  Institution  to  the  rooms  of  the  Royal 
Society,  and  gave  his  first  public  demonstration 
of  its  extraordinary  properties.  On  that  occasion 


THE   LIQUEFACTION   OF  GASES         59 

liquid  hydrogen  flowed  like  water,  and  its  pro- 
duction in  any  quantity  must  now  be  regarded 
simply  as  a  matter  of  expense. 

By  carefully  isolating  a  portion  of  liquid  hydro- 
gen and  preserving  it,  in  a  manner  shortly  to  be 
described,  from  the  access  of  heat  from  without,  it 
was,  when  suddenly  exhausted  under  an  air-pump, 
transformed  into  a  mass  of  solid  frozen  foam.  By 
immersing  a  tube  containing  the  liquid  in  this 
frozen  foam,  a  small  quantity  of  the  clear  trans- 
parent ice  of  solid  hydrogen  was  obtained. 

Kept  in  an  open  vessel,  liquid  air  and  liquid 
hydrogen  are  analogous  to  the  water  in  a 
saucepan  boiling  over  a  fire.  At  the  normal 
atmospheric  pressure,  water  boils  at  100°  C., 
and  the  rate  at  which  it  evaporates  depends 
simply  on  the  rate  at  which  heat  enters  it 
— depends,  that  is  to  say,  on  the  fire  below  it. 
In  a  similar  way,  liquid  air  has  a  definite  boiling- 
point,  which,  under  the  normal  pressure  of  the 
atmosphere,  rises  from  —192°  to  —182°  C.  as 
evaporation  proceeds.  This  rise  is  due  to  the  fact 
that  nitrogen  is  more  volatile  than  oxygen  ;  and 
thus  the  liquid,  as  it  boils  away,  gradually  becomes 
richer  in  oxygen.  Liquefied  air  cannot  be  kept 
in  closed  vessels.  Its  vapour  pressure,  equal  to 
the  pressure  of  the  atmosphere  at  — 190°,  becomes 


60  PHYSICAL  SCIENCE 

enormously  great  as  heat  enters  and  the  tempera- 
ture rises.  In  an  open  vessel,  as  heat  enters 
evaporation  proceeds,  and  the  heat  is  used  to  effect 
the  change  of  state.  Thus,  owing  to  this  latent 
heat  of  evaporation  which  is  absorbed,  no  rise  of 
temperature  (except  the  very  small  change  already 
noted)  occurs.  But,  in  a  closed  vessel,  as  heat 
enters  the  pressure  will  rise,  and  the  jboiling-point 
will  rise  with  it.  The  initial  temperature  being  so 
low  a  large  rise  of  temperature  is  possible,  and  a 
consequent  very  great  increase  in  pressure.  As 
ordinary  temperatures  were  approached  no  vessel 
would  withstand  the  internal  pressure  of  the  air. 

In  order  to  preserve  liquid  air  for  any  time  in 
an  open  vessel,  it  is  clearly  necessary  to  prevent 
as  far  as  possible  the  access  of  heat.  Evapora- 
tion must  always  be  proceeding,  but,  by  dimin- 
ishing the  rate  at  which  it  goes  on,  the  loss  of 
liquid  can  be  considerably  retarded. 

Heat  passes  from  one  place  to  another  in  three 
ways  :  by  conduction,  when  heat  flows  from  one 
part  of  a  body  to  another,  or  between  two  bodies 
in  contact ;  by  convection,  when  air  or  water, 
heated  by  contact  with  a  hot  body,  rise  through 
the  colder  surrounding  fluid,  carrying  heat  with 
them  ;  by  radiation,  when  heat  passes  directly  from 
one  body  to  another,  as  from  the  sun  to  the  earth, 
without  warming  the  intervening  medium.  Bearing 


THE  LIQUEFACTION  OF  GASES         61 

in  mind  these  three  modes  of  transference,  Pro- 
fessor Dewar  has  invented  a  vessel  in  which  a 
liquid  gas  can  be  kept,  and  the  effects  of  all  three 


FIG.  i. 

of   these  methods  of  heat-transfer  be  reduced  to 
a  minimum. 

A  double-walled  glass  bulb  was  taken,  of  one  of 
the  forms  shown  in  Fig.  i,  and  the  space  between 
the  walls  exhausted  of  air  to  the  completest  degree 
possible.  This  arrangement  diminished  the  effects 


62  PHYSICAL  SCIENCE 

of  conduction  and  convection  to  such  an  extent 
that  liquid  air,  placed  within,  evaporated  at  only 
one-fifth  of  the  normal  rate.  An  additional  device 
enabled  the  effects  of  radiation  to  be  diminished 
also.  A  polished  metallic  surface  is  the  worst  radi- 
ator and  the  worst  absorber  of  radiation  known, 
and,  by  coating  the  inner  wall  of  the  vacuum  vessel 
with  a  film  of  bright  silver  or  mercury,  the  rate  of 
evaporation  of  liquid  air  was  again  reduced  to  the 
sixth  part.  By  the  combined  results  of  the  vacuum 
and  the  silvering,  the  rate  of  loss  of  liquid  was  thus 
reduced  to  the  thirtieth  part  of  its  value  in  an 
ordinary  open  vessel.  Without  the  use  of  these 
vessels,  liquid  air  could  never  be  kept  for  any 
length  of  time,  and  liquid  hydrogen,  at  any  rate, 
could  never  have  been  collected  at  all. 

With  the  liquefaction  of  hydrogen  the  old  class 
of  so-called  permanent  gases  disappeared.  In  place 
of  them,  however,  a  number  of  gases,  previously 
unknown  to  science,  have  been  recently  discovered. 
Argon,  shown  by  Lord  Rayleigh  and  Sir  William 
Ramsay  to  exist  in  the  atmosphere,  was  the  first  of 
these  to  be  detected.  Its  name  attempts  to  de- 
scribe its  general  chemical  inertness ;  and  since 
this  discovery  several  other  new  gases  of  some- 
what similar  chemical  properties  have  been 
detected. 


THE  LIQUEFACTION  OF  GASES         63 

The  story  of  the  discovery  and  isolation  of 
argon  is  an  excellent  example  of  the  importance 
in  science  of  the  infinitely  little,  and  shows  how 
striking  discoveries  may  be  made  as  a  conse- 
quence of  experiments  which  seem  at  first  sight 
simply  adapted  to  investigate,  with  the  greatest 
attainable  accuracy,  phenomena  already  known  to 
science.  Since  the  days  of  Cavendish,  the  com- 
position of  the  air  had  been  looked  upon  as  an 
ascertained  fact ;  a  certain  proportion  had  been 
shown  to  be  oxygen,  varying  amounts  of  carbonic 
acid  and  aqueous  vapour  were  known  to  be  present, 
while  the  remainder,  as  the  result  of  careful  in- 
vestigation, was  supposed  to  be  nitrogen.  Caven- 
dish himself  knew,  so  accurate  was  his  work, 
that  any  undetected  residue  could  not  exceed 
the  TJ-0-th  part.  But  in  the  course  of  a  long  series 
of  experiments,  undertaken  to  determine  afresh 
the  densities  of  the  principal  gases,  Lord  Ray- 
leigh  detected  a  slight  difference  in  the  density 
of  nitrogen  as  prepared  from  ammonia  and  as 
extracted  from  the  air.  This  difference,  amount- 
ing at  first  to  about  o.i  per  cent.,  was  increased 
on  subsequent  more  careful  examination  to  nearly 
a  half  per  cent.  It  was  clear  that  the  gases  pre- 
pared by  these  two  methods  were  not  identical, 
and  that  some  hitherto  unknown  body  was  re- 
sponsible for  the  complication.  The  existence  of 


64  PHYSICAL  SCIENCE 

this  new  body,  the  inert  gas  now  known  as  argon, 
was  announced  by  Rayleigh  and  Ramsay  in  1894, 
and  shortly  afterwards  it  was  isolated  from  its 
companion. 

Argon  is  slightly  more  soluble  in  water  than 
nitrogen,  hence  a  rather  larger  proportion  of  it 
than  might  be  expected  is  found  in  rain  water. 
It  is  also  contained  to  a  small  extent  in  the  gases 
liberated  from  certain  thermal  springs.  Recently 
traces  of  three  other  gases,  neon,  krypton,  and 
xenon,  which  much  resemble  argon  in  chemical 
properties,  have  been  detected  in  the  atmosphere. 
The  total  amount  of  these  three  substances  is  almost 
immeasurably  small,  and  does  not  altogether  exceed 
the  four-hundredth  part  of  the  argon  present. 

The  spectrum  of  the  sun  shows  lines  which  do 
not  coincide  with  those  of  any  chemical  element 
under  conditions  usually  known  on  the  earth. 
Among  these  lines  many  are  due  to  terrestrial 
elements  in  solar  circumstances,  but  a  bright  line 
in  the  yellow  part  was  detected  in  the  spectrum 
of  a  solar  prominence,  and  was  examined  carefully 
by  Frankland  and  Lockyer  during  the  eclipse  of 
August  1868.  To  explain  its  presence  they  called 
into  existence  a  hypothetical  element,  placed  it  in 
the  sun,  and  gave  to  it  the  name  helium.  For 
many  years  the  line  in  the  sun's  spectrum  was 
the  only  evidence  for  the  existence  of  helium ; 


THE  LIQUEFACTION  OF  GASES         65 

but  in  1895  its  presence  on  the  earth  was  an- 
nounced by  Ramsay,  who  had  detected  it  in  the 
spectroscopic  analysis  of  the  gases  dissolved  in 
the  mineral  clevite,  together  with  the  other  new 
gases  krypton  and  neon.  Since  this  discovery, 
helium  has  been  isolated  and  collected  in  appreci- 
able quantities,  and  its  physical  and  chemical 
properties  are  now  well  known.  Helium  alone, 
of  all  substances  investigated,  has  not  be 
liquefied.  It  has  been  expanded  from  a  pressure 
of  100  atmospheres  at  the  temperature  of  solid 
hydrogen  with  no  signs  of  liquefaction,  and 
has  resisted  hitherto  all  the  refrigerating  resources 
of  the  Royal  Institution. 

It  will  be  seen  from  the  foregoing  account  that 
the  difficulty  of  obtaining  these  low  temperatures 
is  very  great.  While  a  temperature  of  many 
hundred  degrees  above  the  freezing-point  of  water 
is  easily  reached  in  a  common  fire  or  gas  flame, 
to  cool  hydrogen  to  250°  below  that  point  needs 
the  use  of  powerful  engines,  of  elaborate  and  costly 
apparatus.  The  difference  is  very  marked.  More- 
over, it  becomes  more  and  more  difficult  to  cool 
a  substance  through  one  degree  as  we  pass  down 
the  scale.  This  fact  suggests  that  there  is  some 
lower  limit  of  temperature  towards  which  we  may 
strive,  but  with  the  prospect  of  encountering  in- 


3/TY 


66  PHYSICAL  SCIENCE 

creasing  difficulty  as  we  approach ;  it  suggests, 
that  is  to  say,  the  existence  of  an  absolute  zero 
of  temperature. 

Our  knowledge  of  an  absolute  scale  of  tem- 
perature is  due  to  the  genius  of  Lord  Kelvin,  who, 
with  Clausius,  Rankine,  and  Helmholtz,  may  be 
said  to  have  founded  the  modern  science  of 
thermodynamics  about  the  year  1850.  It  may  be 
shown  that  Lord  Kelvin's  absolute  scale  of  tem- 
perature coincides  with  the  scale  of  an  ideal  gas — 
a  gas,  that  is,  such  as  air  would  be  if  its  molecules 
exerted  no  forces  on  each  other,  and,  conse- 
quently, its  porous  plug  effect  were  nil.  As 
a  matter  of  fact,  at  ordinary  temperatures  and 
pressures,  such  gases  as  air  or  hydrogen  conform 
very  nearly  to  these  conditions — so  nearly  that, 
for  all  ordinary  purposes,  their  deviations  may 
be  neglected.  Now,  if  we  keep  a  gas  at  constant 
pressure,  its  volume  changes  from  i  to  1.366.  as  it 
is  heated  from  the  freezing  to  the  boiling-point 
of  water.  Similarly,  if  it  be  kept  at  constant 
volume,  its  pressure  increases  in  the  same  ratio. 
If  we  use  either  of  these  changes  as  our  thermo- 
metric  property,  and  divide  the  interval  between 
the  freezing  and  boiling-points  into  100°  in  the 
Centigrade  manner,  there  will  be  a  change  in 

pressure,  for  example,  of  0.00366  or  of    the 


Photo  by  II  -indtnv  &  Grove 


To  face  page  66 


THE   LIQUEFACTION   OF  GASES         67 

pressure  at  o°,  for  each  degree  through  which  the 
gas  is  heated.     If  we  call  the  pressure  at  o°  unity, 

then  at  i°  the  pressure  will  be  I  +  7r,  a*  2°  it  will 


be  i+  —  ,  and  so  on.  Similarly,  if  we  cool  the 
gas  below  the  freezing-point,  at  —  1°  the  pressure 

becomes    i  —  —  ,  at   —2°  the  pressure    is   i  --  - 

If,  while  we  carry  on  this  process,  the  properties 
of  the  gas  remain  unchanged,  as  they  would  were 
it  the  ideal  gas  we  have  supposed,  at  a  temperature 


of  —273°  the  pressure  will  fall  to  i  —  —  -,    that    is, 

i  —  i,  or  zero.  At  —273°  C.,  therefore,  the  pres- 
sure of  an  ideal  gas  would  vanish  absolutely, 
and  no  further  cooling  could  make  it  smaller. 
On  the  temperature  scale  which  uses  the  pressure 
of  an  ideal  gas  as  the  thermometric  property, 
—  273°  C.  represents  an  absolute  zero,  the  lowest 
conceivable  degree  of  cold.  But,  as  we  said,  such 
a  scale  coincides  exactly  with  the  true  absolute 
or  thermodynamic  scale,  and,  as  can  be  shown, 
unlike  all  other  temperature  scales,  is  independent 
of  the  properties  of  any  particular  substance, 
whether  real  or  imaginary.  On  the  thermo- 
dynamic scale  also,  then,  —273°  C.  represents 
the  absolute  zero. 

We  thus  see  that  the  idea   of  an  absolute  zero, 


68  PHYSICAL  SCIENCE 

at  which  all  bodies  would  be  entirely  deprived  of 
heat  energy,  is  not  a  mere  figment  of  the  mathe- 
matical imagination,  derived  from  the  study  of  a 
hypothetical  air  thermometer.  It  has  a  real  phy- 
sical meaning,  and  the  attainment  of  the  absolute 
zero  is,  at  all  events,  theoretically  possible. 

From  the  practical  side,  however,  difficulties 
accumulate  and  increase  as  the  absolute  zero  is 
approached.  As  Professor  Dewar  has  remarked, 
"  the  step  between  the  liquefaction  of  air  and  that 
of  hydrogen  is,  thermodynamically  and  practically, 
greater  than  that  between  the  liquefaction  of 
chlorine  and  that  of  air."  The  boiling-points  of 
chlorine,  air,  and  hydrogen  under  the  atmos- 
pheric pressure  are  —33°,  —193°  and  —253°  C.  re- 
spectively. If  we  express  these  temperatures  on 
the  absolute  scale,  they  become  240°,  80°  and  20°. 
The  interval  between  the  boiling-points  of  chlorine 
and  air  is  160°,  but  the  ratio  of  the  absolute  tem- 
peratures is  240  :  80,  or  3  :  i.  On  the  other  hand, 
while  the  interval  between  air  and  hydrogen  is 
only  60°,  the  ratio  of  the  absolute  temperatures  is 
80  :  20,  or  4  :  i.  The  difficulty  of  the  transition 
from  one  to  the  other  temperature  is  much  more 
nearly  proportional  to  the  ratio  than  to  the  dif- 
ference between  them. 

The  absolute  boiling-point  of  hydrogen  is,  as  we 
have  said,  about  20°,  and  at  present  this  tern- 


THE  LIQUEFACTION   OF  GASES         69 

perature  is  the  lowest  at  which  we  can  continuously 
keep  a  body  long  enough  to  examine  its  properties. 
Any  further  advance  towards  the  absolute  zero 
must  be  made  by  the  help  of  helium.  By  the 
sudden  expansion  of  helium  at  a  pressure  of  100 
atmospheres  and  at  the  temperature  of  solid  hy- 
drogen, it  is  estimated  that  a  transient  temperature 
of  9°  or  10°  absolute  has  been  reached.  Since  the 
gas  was  not  liquefied,  the  inference  is  that  its  cri- 
tical point  is  below  9°,  and  its  normal  boiling- 
point  about  5°  absolute.  This  is  about  one-fourth 
that  of  hydrogen,  and  it  will,  therefore,  be  at  least 
as  hard  to  pass  the  interval  between  hydrogen 
and  helium,  as  it  was  to  pass  that  from  air  to 
hydrogen. 

Nevertheless,  it  is  likely  that  success  is  merely  a 
matter  of  expense  and  labour.  By  the  regenerative 
cooling  of  helium,  previously  cooled  by  liquid 
hydrogen  boiling  under  diminished  pressure,  a 
temperature  lower  than  any  yet  attained  could  be 
reached,  and  it  is  probable  that,  before  the  lapse 
of  many  years,  the  last  known  "  permanent " 
gas  will  yield  to  Professor  Dewar's  skill  and 
perseverance. 

The  liquefaction  of  helium  will  give  command  of 
a  steady  temperature  of  about  5°  absolute,  which  is 
its  probable  boiling-point  in  open  vessels.  That 
temperature,  within  5°  of  the  absolute  zero,  is  thus 


70  PHYSICAL  SCIENCE 

almost  within  sight ;  but  there,  with  our  present 
methods  and  materials,  seems  to  come  the  end  of 
any  possible  advance.  If,  in  the  future,  a  new  gas, 
similar  to  helium  but  of  less  density,  should  be 
discovered,  we  should  find  that  it  was  still  more 
difficult  to  liquefy.  By  using  liquid  helium  as  a 
means  of  preliminary  cooling,  the  resistance  of 
this  hypothetical  gas  may  possibly  be  overcome, 
and,  by  collecting  it  in  open  vessels  under  the 
atmospheric  pressure,  a  steady  temperature  of  i° 
or  2°  absolute  may  some  day  be  placed  at  the 
disposal  of  the  physicist. 

With  this  forecast,  the  present  brief  historical 
account  of  low  temperature  research  will  be  closed 
that  attention  may  be  drawn  to  the  methods  of 
measuring  the  temperatures  therein  obtained. 

Mercury  freezes  at  a  temperature  of  —  40°  C., 
and,  at  such  temperatures  as  those  now  under 
consideration,  a  mercury  thermometer  clearly  is 
useless.  The  resistance  of  a  metallic  wire  to  the 
passage  of  an  electric  current  is  a  quantity  which 
can  be  measured  easily  and  accurately.  This  re- 
sistance, diminishing  as  the  wire  is  cooled,  depends 
on  the  temperature.  With  some  alloys  the  diminu- 
tion of  resistance  with  temperature  is  very  small, 
but  with  pure  metals  it  is  considerable,  and 
roughly,  at  any  rate,  proportional  to  the  change 


THE  LIQUEFACTION  OF  GASES         71 

of  temperature.  The  metal  most  usually  employed 
is  platinum,  since  it  is  not  attacked  by  acids,  and 
is  very  infusible.  Platinum  resistance  thermo- 
meters are  now  extensively  used  for  physical 
research  ;  they  have  a  very  large  range,  and  are 
probably  susceptible  of  greater  sensitiveness  than 
any  other  form  of  thermometer.  At  ordinary  tem- 
peratures a  difference  of  temperature  of  one  ten- 
thousandth  of  a  degree  can  be  detected  with 
moderate  ease,  while,  with  great  precautions,  the 
hundred-thousandth  of  a  degree  can  be  estimated. 
At  high  or  low  temperatures  such  accuracy  is  im- 
possible, but  measurements,  correct  to  the  nearest 
degree,  can  be  made  up  to  about  1100°  C. 
and  as  low  as  —200°  C.  Below  the  latter  tem- 
perature the  rate  of  change  of  the  resistance  alters 
in  a  manner  not  yet  fully  investigated,  and  the 
instrument  ceases  to  be  trustworthy. 

The  standard  to  which  the  readings  of  all  other 
thermometers  are  referred,  as  we  have  indicated 
when  considering  the  absolute  scale  of  tempera- 
ture, is  the  gas  thermometer  containing  nitrogen 
or  hydrogen.  Not  only  is  the  hydrogen  thermo- 
meter thus  used  for  purposes  of  reference,  but 
it  can  also  be  employed  as  a  practical  instru- 
ment at  temperatures  too  low  to  be  measured 
by  the  platinum  resistance  thermometer.  It 
might  be  thought  that,  as  the  point  of  lique- 


72  PHYSICAL  SCIENCE 

faction  was  approached,  a  gas  would  cease  to 
be  trustworthy  as  a  thermometric  substance, 
but  experiment  has  shown  that,  as  long  as 
the  pressure  of  the  gas  is  kept  well  below  the 
saturation  value  at  which  condensation  would 
occur,  the  gas  still  expands  or  contracts  propor- 
tionally to  the  absolute  temperature.  Dewar  has 
found  that  thermometers,  filled  with  oxygen  and 
carbonic  acid  at  low  pressures,  gave  correct  tem- 
peratures as  low  as  the  boiling-points  of  those 
gases  at  the  normal  atmospheric  pressure.  He 
used  therefore  a  constant  volume  hydrogen  ther- 
mometer, working  at  low  pressure,  to  determine 
the  boiling-point  of  liquid  hydrogen  itself,  and 
confirmed  the  result  obtained,  —  252°  C.,  by  ex- 
periments with  a  similar  thermometer  filled  with 
helium. 

Some  very  remarkable  effects  are  obtained  with 
liquid  hydrogen.  A  vessel  containing  it  is  so 
cold  that  the  air  in  contact  with  it  immediately 
freezes.  A  snow-shower  of  solid  air  is  thus  pro- 
duced. This  process  may  be  applied  to  the  pro- 
duction of  very  high  vacua.  If  the  vessel  to  be 
exhausted  be  sealed  to  a  long  tube,  one  end  of 
which  is  plunged  into  liquid  hydrogen,  the  air  in 
the  vessel  is  almost  completely  frozen  out.  The 
air  in  the  cooled  end  of  the  tube  first  condenses, 


THE  LIQUEFACTION  OF  GASES         73 

but;  as  it  is  removed,  the  residual  air  in  the  vessel 
expands,  again  fills  the  whole  tube,  and  again  that 
portion  of  it  in  contact  with  the  cold  part  of 
the  tube  is  frozen.  This  process  continues  till  the 
pressure  within  the  tube  falls  to  the  millionth  of 
an  atmosphere  or  less,  a  pressure  so  low  that  an 
electric  discharge  will  only  pass  through  the  vessel 
with  extreme  difficulty.  If,  instead  of  air,  the 
vessel  contains  some  more  condensible  gas,  such 
as  sulphur  dioxide  or  water  vapour,  the  same  effect 
can  be  produced  by  the  use  of  liquid  air  as  the 
condensing  agent. 

The  liquefaction  of  air  and  hydrogen  has  enabled 
many  experiments  to  be  made  on  the  influence  of 
low  temperature  on  chemical  action,  and  it  is  found 
that  the  rate  of  change  is  very  greatly  affected  at 
these  temperatures.  In  many  cases,  where  the 
reaction  proceeds  rapidly  at  ordinary  tempera- 
tures, the  rate  is  reduced  to  such  an  extent  that 
in  liquid  air  it  becomes  too  small  to  be  observed. 
In  other  cases  action  may  cease  altogether,  and 
reagents  which  would  otherwise  undergo  chemi- 
cal change  are  maintained  in  false  equilibrium  by 
chemical  forces  analogous  to  those  of  friction. 
Fluorine,  for  instance,  which  attacks  glass  violently 
at  ordinary  temperatures,  has  no  effect  on  it  when 
cooled  to  -180°  C. 

As  yet,  for  the  purposes  of   physical  research, 


74  PHYSICAL  SCIENCE 

a  beginning  only  has  been  made  in  the  use  of  the 
low  temperatures  now  at  our  disposal.  Neverthe- 
less, Dewar,  as  well  as  Dewar  and  Fleming  working 
together,  have  already  obtained  some  interesting 
results.  It  is  found  that  the  elasticity  of  materials 
is  greatly  affected  by  these  low  temperatures.  On 
the  one  hand,  iron,  lead,  and  tin,  as  well  as  ivory, 
showed  a  considerable  increase  in  this  property, 
balls  of  these  substances  rebounding  to  a  much 
greater  height  than  usual.  On  the  other  hand,  a 
ball  of  india-rubber  became  brittle,  and  was  broken 
by  the  fall.  Connected  with  the  increase  in  the 
elasticity  of  metals  is  their  increased  strength ; 
wires,  for  example,  will  stand  a  much  greater 
load  without  pulling  out  or  breaking. 

Low  temperatures  also  affect  the  magnetic  pro- 
perties of  iron,  cobalt,  and  other  metals,  which  are 
usually  magnetic  at  ordinary  temperatures,  gene- 
rally increasing  the  magnetic  moment.  Oxygen, 
slightly  magnetic  as  a  gas,  as  a  liquid  becomes 
strongly  magnetic.  The  alteration  of  magnetic 
properties  with  temperature  has  been  studied  in 
detail  for  many  years  where  high  temperatures 
are  concerned,  and  this  extension  of  the  research 
has  been  of  great  interest. 

From  the  point  of  view  of  the  popular  lecture- 
room,  some  of  the  prettiest  effects  given  by  liquid 


THE   LIQUEFACTION  OF  GASES         75 

air  depend  on  its  power  of  imparting  phosphores- 
cence to  many  substances  which  do  not  usually 
possess  this  property.  Ivory,  egg-shells,  paper, 
cotton-wool,  and  many  other  things  glow  brightly 
in  liquid  air  after  they  have  been  exposed  to  light. 
On  the  other  hand,  certain  sulphides  of  calcium, 
phosphorescent  at  ordinary  temperatures,  cease  to 
be  so  when  cooled.  Some  crystals,  such  as  those 
of  uranium  nitrate,  become  self-luminous  in  liquid 
hydrogen,  apparently  owing  to  intense  electric 
forces  set  up  by  the  cooling.  These  forces  may 
become  so  intense  that  discharges  take  place 
which  are  powerful  enough  to  be  visible  in  the 
dark. 

It  will  be  seen  from  this  account  that  the  changes 
in  the  properties  of  matter  are  more  striking  and 
complete  in  the  range  of  temperature  below  the 
freezing-point  of  water  than  in  the  correspond- 
ing range  of  temperature  above  that  point.  The 
difference  in  the  character  and  intensity  of  these 
changes  emphasises  the  importance  of  the  ratios 
of  temperature  as  measured  on  the  thermodynamic 
or  absolute  scale,  and  accounts  for  the  greatly  in- 
creased difficulty  of  manipulation  created  by  every 
nearer  approach  to  the  absolute  zero.  On  the 
other  hand,  it  is  very  striking  that  in  biological 
problems,  more  especially  in  those  connected  with 


76  PHYSICAL  SCIENCE 

the  lowliest  forms  of  animal  and  vegetable  life,  a 
hundred  degrees  above  the  freezing-point  is  pro- 
ductive of  a  more  complete  and  destructive  change 
than  a  hundred  degrees  below.  While  exposure  to 
the  boiling-point  of  water,  or  to  a  temperature  a 
few  degrees  higher,  suffices  to  kill  all  known  forms 
of  living  organisms,  many  forms  of  bacteria  merely 
have  their  vitality  temporarily  suspended  in  liquid 
air.  Even  seeds  of  barley,  peas,  &c.,  were  not 
permanently  affected ;  in  fact,  they  have  recently 
been  placed  for  six  hours  in  liquid  hydrogen 
with  no  effect  on  their  subsequent  power  of  ger- 
mination. 

In  closing  this  account  of  low  temperature 
research  it  may  be  of  interest  to  tabulate  some 
of  the  more  important  temperature-constants  now 
known  to  mankind.  In  doing  so,  we  cannot  fail 
again  to  be  struck  by  the  high  temperatures  easily 
obtainable.  On  the  other  hand,  to  cool  an  object 
through  250°  of  the  273°  which  separates  the 
freezing-point  of  water  from  the  absolute  zero  has 
taxed  the  skill  of  experimenters  for  several  genera- 
tions. Temperatures,  as  already  pointed  out, 
are  more  justly  compared  by  considering  their 
ratio  on  the  absolute  scale  than  by  consider- 
ing the  number  of  degrees  Centigrade  or  Fahren- 
heit which  separate  them. 


THE   LIQUEFACTION   OF  GASES         77 


Temperature. 

On  Absolute 

On  Centigrade 

Scale. 

Scale. 

Zero  of  the  absolute  scale  . 

O° 

-273° 

Lowest  temperature  yet  reached 
Boiling-point  of  liquid  hydrogen 

10° 
20° 

-263° 

-253° 

Critical-point  of  hydrogen 

3°° 

-243° 

Boiling-point  of  liquid  air 

8i°to9i° 

-  192°  to  -  182° 

Boiling-point  of  liquid  carbonic  acid 

195° 

-78° 

Freezing-point  of  water     . 

273° 

0° 

Boiling-point  of  water 

273° 

100° 

Melting-point  of  tin           ... 
Melting-point  of  lead 

5050 
601° 

23i°-7 
327°-  7 

Boiling-point  of  sulphur    . 

718° 

444°-  5 

Melting-point  of  silver 

1234° 

960°.  7 

Melting-point  of  gold 

1335° 

io6i°.7 

Melting-point  of  copper     . 
Melting-point  of  platinum 

1354° 
2073° 

1  080°.  5 
1800° 

Low  red  heat     .... 
White  heat         .        . 
Temperature  of  furnace 
Temperature  of  electric  arc 
Estimated  temperature  of  the  sun 


Approximate  Temperature 
on  Centigrade  Scale. 
500°  to  600° 
1500°  to  1800° 
1500°  to  1600° 
3000°  to  4000° 
5700°  to  7000° 


CHAPTER  III 

FUSION   AND   SOLIDIFICATION 

"  For  more  is  not  reserved 

To  man,  with  soul  just  nerved 
To  act  to-morrow  what  he  learns  to-day  : 
Here  work  enough  to  watch 
The  Master  work  and  catch 

Hints  of  the  proper  craft,  tricks  of  the  tool's  true  play." 
— BROWNING,  Rabbi  Ben  Ezra. 

IN  the  previous  chapter  we  have  discussed  chiefly 
the  methods  employed  to  bring  about  a  change  of 
state,  especially  that  change  of  state  which  consists 
in  passing  from  the  gaseous  to  the  liquid  or  solid 
condition  in  the  case  of  those  substances  which  at 
ordinary  temperatures  and  pressures  exist  as  gases. 
The  methods  employed  and  the  principles  under- 
lying them  were  the  points  of  interest,  and  the 
whole  subject  belonged  to  that  branch  of  physical 
science  which  consists  in  recognising  and  over- 
coming difficulties  of  manipulation,  and,  as  it  were, 
of  asserting  by  force  the  superiority  of  mind  over 
matter. 

But,  throughout  the  investigations  to  be  pursued 
in  the  present  chapter,  our  attitude  is  altered. 
There  is  no  need  for  such  attempted  assertion  of 


FUSION   AND   SOLIDIFICATION          79 

supremacy.  The  changes  of  state  to  be  examined 
are  already  under  our  control,  and  we  are  able  to 
investigate  further  details,  and  probe  more  deeply 
into  the  intimate  nature  of  the  processes  involved. 
We  patiently  seek  to  trace  connections  between, 
for  example,  the  mechanical  properties  of  metals 
and  their  microscopic  structure  when  solidified  ; 
and,  from  the  complicated  relations  which  declare 
themselves,  we  may  hope  to  throw  light  on  the  pro- 
cesses of  fusion  and  solidification,  and  construct  a 
theory  that  will  hereafter  prove  of  some  use  to  the 
engineer  and  the  metallurgist. 

In  the  first  place  it  is  well  to  remark  that  we  are 
seldom  dealing  with  pure  materials.  Nearly  the 
whole  of  the  phenomena  we  shall  consider  depend 
on  the  admixture  of  two  or  more  substances,  one 
for  the  most  part  predominating.  It  follows  that 
the  result  of  the  inquiry  is  specially  applicable  in 
all  cases  where  traces  of  some  impurity  are  the 
determining  factor ;  that  is,  to  the  majority  of 
cases,  since  the  attainment  of  chemical  purity  is 
more  often  a  pious  hope  than  an  accomplished 
fact. 

Our  investigations  will  lead  us  far  afield,  and  we 
shall  pass  in  review  combinations  of  many  of  the 
principal  metals.  It  is  well,  however,  that  the 
starting-point  should  be  on  familiar  ground  ;  if, 
indeed,  by  such  a  term  it  is  permissible  to  indicate 


8o  PHYSICAL  SCIENCE 

the   ice   that   occasionally   covers   our   ponds  and 
perpetually  caps  our  globe. 

It  is  well  known  that  sea-water  remains  liquid  at 
temperatures  low  enough  to  freeze  ponds  and 
lakes,  and,  long  ago,  it  must  have  been  recognised 
that  this  behaviour  was  due  to,  the  dissolved  salt, 
though  it  was  not  till  the  year  1788  that  Blagden, 
the  first  worker  in  the  field,  published  a  systematic 
series  of  observations  on  the  freezing-points  of  salt 
solutions. 

If  we  cool  the  solution  of  some  substance  such 
as  sodium  chloride,  that  is,  common  salt,  the  ice 
which  freezes  out  is  the  solid  form  of  pure  water. 
The  process  can  be  illustrated  in  a  very  striking 
manner  by  using  the  solution  of  a  coloured  salt. 
If,  for  example,  a  dilute  solution  of  the  purple- 
coloured  potassium  permanganate  be  placed  in  a 
glass  bottle  and  be  surrounded  for  some  hours  by 
a  freezing  mixture,  most  of  the  water  solidifies 
to  form  a  hollow  cylinder  of  perfectly  colourless 
ice,  while  the  permanganate  is  concentrated  in  an 
intensely  coloured  liquid  core  along  the  axis  of  the 
cylinder. 

Similar  phenomena  occur  in  other  cases  where 
the  separation  is  not  so  clearly  visible. 

If  the  ice  be  frozen  rapidly,  some  trace  of  salt  may 
be  deposited  also  ;  but  experiment  has  shown  that 


FUSION  AND   SOLIDIFICATION          81 

it  does  not  enter  into  the  composition  of  the 
crystals,  and  is  entangled  merely  mechanically  in 
their  interstices.  Essentially,  then,  the  salt  remains 
in  the  liquid  solution,  and,  as  the  solvent  is  gradu- 
ally frozen  out,  the  concentration  of  that  solution 
must  increase.  The  stronger  the  solution  becomes 
the  lower  is  its  freezing-point ;  but,  if  the  tem- 
perature at  our  disposal  is  low  enough,  we  can 
go  on  freezing  out  water  till  the  residual  solution 
is  saturated  with  salt  at  the  temperature  of  its 
freezing-point.  Any  further  abstraction  of  heat, 
by  removing  some  of  the  necessary  solvent,  must 
then  be  accompanied  by  the  simultaneous  deposi- 
tion of  salt ;  ice  and  salt  will  be  precipitated 
together,  and  the  residual  solution  will  retain  the 
constant  composition  of  saturation. 

Since,  as  the  process  of  freezing  goes  on  in 
these  conditions,  there  is  no  change  in  the 
composition  of  the  residual  liquid,  there  can 
be  no  change  in  the  freezing-point.  The 
mixture  of  salt  and  water  of  this  particular  con- 
centration will  solidify  completely  at  a  constant 
temperature  into  a  mixture  of  salt  and  ice  of  the 
same  composition.  But  pure  chemical  elements 
like  lead,  or  pure  compounds  like  water,  also  fuse 
and  solidify  at  constant  temperatures  without 
change  of  composition.  In  these  respects,  then, 
the  particular  mixture  of  salt  and  water  which  we 


82  PHYSICAL  SCIENCE 

are  considering  behaves  like  a  pure  element  or 
compound.  For  this  reason  Guthrie,  who  first 
systematically  examined  such  mixtures,  classed 
them  as  compounds,  and  named  them  cryo- 
hydrates.  It  is,  however,  now  evident  that  their 
properties  are  explicable  in  other  ways. 

The  phenomena  we  have  traced,  and  the  exist- 
ence of  a  cryohydric  point,  must  be  borne  in  mind 
if  we  wish  to  understand  the  structure  of  natural 
ice,  the  properties  of  metallic  alloys,  or  the  pro- 
cesses which  occur  when,  in  the  cold  of  an  Arctic 
winter,  sea-water  becomes  coated  with  a  solid 
covering. 

Natural  waters,  even  when  known  as  fresh,  con- 
tain some  amount  of  solids  in  solution.  When 
such  waters  are  cooled  to  the  freezing-point,  how- 
ever, the  crystals  which  appear  form  the  ice  of 
pure  water.  As  the  crystals  grow,  the  dissolved 
salts  become  concentrated  into  the  liquid  which 
remains ;  and  the  freezing-point  of  this  liquid  falls 
as  its  concentration  rises.  Unless  the  temperature 
of  the  cryohydric  point  is  reached,  some  liquid 
must  always  remain,  though,  with  fairly  pure  water, 
it  may  exist  only  as  a  thin  film  between  the  solid 
crystals.  If  the  temperature  sink  below  the  cryo- 
hydric point,  these  liquid  films  themselves  solidify ; 
but,  even  then,  the  mass  is  not  a  homogeneous 
solid,  for  the  cryohydric  conglomerate  forms  a 


Fro.  2 
Magnification  45 


FIG.  3 
Magnification  200 


FIG.  4 
Magnification  50 


FIG.  5 
Magnification  120 


To  face 


FUSION  AND  SOLIDIFICATION          83 

cement-like  connection  between  the  primary  crys- 
tals of  pure  ice.  We  see  now  the  explanation  of 
the  fact  that  a  block  of  natural  ice,  taken  from  a 
glacier  or  lake,  has  a  definite  structure,  and  may 
be  resolved  into  a  heap  of  separate  crystals  by 
exposure  to  the  sun.  The  cryohydric  cement 
dissolves  first  at  the  lower  temperature,  and  thus 
the  primary  crystals  of  pure  ice  fall  away  from 
each  other  before  the  temperature  rises  to  their 
melting-point. 

Phenomena  precisely  similar  to  those  we  have 
described  appear  when  a  fused  metal  is  allowed  to 
solidify.  Crystalline  structures  of  pure  metal  form 
in  the  liquid,  and  grow  till  the  whole  mass  becomes 
solid.  These  primary  crystals  usually  start  as  fern- 
like  forms,  of  which  a  beautiful  example  is  shown 
in  Fig.  2.  This  represents  the  microscopic  struc- 
ture of  a  bronze  ingot,  suddenly  chilled  from  a 
temperature  of  644°  C.  If  the  crystals  be  allowed 
to  grow  by  very  slow  cooling,  they  may  come  to 
fill  nearly  the  whole  mass,  as  in  the  case  of  the 
section  of  iron  shown  in  Fig.  3.  Even  in  this 
case,  with  a  substance  nearly  as  pure  as  can  be 
obtained,  the  lines  of  separation  between  the 
primary  crystals  are  clearly  visible  ;  the  primary 
crystals  are  differently  orientated,  and  their  faces 
reflect  the  incident  light  at  different  angles.  The 


84  PHYSICAL  SCIENCE 

crystals  of  zinc  are  often  remarkably  large  and 
well-defined,  and  fine  specimens  can  be  seen  on 
surfaces  of  so-called  galvanised  iron,  such  as  is 
used  for  water-cisterns,  &c.  When,  instead  of  a 
single  metal,  traces  of  others  are  present,  the  lines 
of  separation  between  the  primary  crystals  are 
much  emphasised,  and,  when  the  quantity  of  other 
substances  is  considerable,  there  arise  the  com- 
plicated structures,  which  we  shall  presently  study 
under  the  head  of  alloys. 

The  process  of  the  freezing  of  sea-water  under 
the  influence  of  the  intense  cold  of  an  Arctic 
climate  is  an  interesting  example  of  the  application 
of  the  same  principles.  The  phenomena  have 
been  described  by  the  explorer,  Weyprecht,  whose 
account  is  quoted  by  Mr.  J.  Y.  Buchanan  in  his 
"  Chemical  and  Physical  Notes."  When  a  new 
surface  of  sea-water  is  exposed  to  the  cold  air, 
in  a  short  time  the  surface  of  the  water  begins  to 
get  thick,  threads  like  a  spider's  web  running  out 
from  the  old  ice.  Brine  is  entangled  in  this  struc- 
ture, and  its  concentration  constantly  becomes 
greater  as  the  quantity  of  ice  increases.  At  this 
stage  the  ice  is  a  pasty  mass,  and  follows  every 
motion  of  the  water  on  which  it  floats.  With 
a  temperature  of  —  40°  C.  the  new  ice,  even 
after  twelve  hours,  is  still  so  soft  that,  in  spite  of 
its  thickness,  a  stick  can  easily  be  thrust  through  it. 


FUSION  AND  SOLIDIFICATION          85 

As  soon  as  a  layer  of  ice  is  formed  over  the  sur- 
face, the  cooling  of  the  underlying  water  proceeds 
much  more  slowly,  and  less  salt  is  entangled  in  the 
crystals.  The  lower  layers  of  sea-water  ice  give 
therefore,  when  melted,  a  much  fresher  water  than 
can  be  obtained  from  the  upper  layers.  Even 
when  strong  enough  to  walk  on,  the  surface  of 
new  sea-ice,  frozen  by  air  at  —40°,  is  still  moist  and 
soft,  the  residual  liquid  consisting  of  a  concen- 
trated solution  of  various  salts,  chiefly  calcium 
chloride.  The  cryohydric  point  of  calcium  chloride, 
an  extremely  soluble  substance,  is  very  low,  and 
that  of  a  mixture  of  salts  will  be  lower  than  that 
of  either  component.  This  lowering  of  the  cryo- 
hydric temperature,  which  corresponds  to  the 
lowering  of  the  freezing-point  of  water  by  the 
addition  of  salt,  was  observed  by  Buchanan  in 
experiments  conducted  in  the  Engadine. 

So  far  the  components  of  the  system  we  have 
been  considering  are  not  miscible  with  each  other 
in  all  proportions  ;  only  a  limited  amount  of  salt 
can  be  dissolved  in  a  given  quantity  of  water 
A  system  not  subject  to  any  such  restriction,  in 
which  the  phenomena  are  as  simple  as  possible,  is 
found  in  mixtures  of  the  metals  silver  and  copper. 
The  equilibrium  of  these  substances  has  been 
studied  by  Mr.  C.  T.  Heycock  and  Mr.  F.  H. 


86  PHYSICAL  SCIENCE 

Neville,  who  have  determined  the  melting-points,  or 
rather  the  points  of  solidification,  of  mixtures  of 
various  proportions  of  the  two  metals.  At  the  high 
temperatures  involved,  it  would,  of  course,  be 
impossible  to  use  a  mercury  thermometer,  and  the 
measurements  were  consequently  made  by  means 
of  a  platinum  resistance  thermometer,  with  which 
the  temperature  is  determined  by  observing  the 
electrical  resistance  of  a  coil  of  platinum  wire.  The 
metals  in  the  required  proportion  are  fused  in  a 
crucible  and  allowed  to  cool.  As  soon  as  solidifica- 
tion sets  in,  the  rate  at  which  the  temperature  falls 
always  becomes  less  ;  and,  in  the  case  of  pure  metals 
and  other  systems  where  the  solid  has  the  same 
composition  as  the  liquid,  the  temperature  remains 
constant  till  solidification  is  complete,  just  as  the 
temperature  of  a  mass  of  ice  and  water  remains 
constant  till  the  whole  is  frozen.  Thus,  by  watch- 
ing the  thermometer,  the  temperature  at  which 
solid  begins  to  form  can  be  estimated. 

The  melting-point  of  silver  is  960°,  C.  and 
the  addition  of  copper  lowers  it  just  as  the  addition 
of  salt  lowers  the  freezing-point  of  water.  This  is 
best  shown  by  plotting  the  observations  on  a 
diagram,  as  in  Fig.  6,  in  which  the  horizontal 
axis  denotes  the  composition  of  the  mixture 
expressed  in  percentage  numbers  of  atomic 
equivalents  of  silver  and  copper,  and  the  verti- 


FUSION  AND  SOLIDIFICATION          87 

cal  axis  the  temperatures.  On  the  other  hand, 
pure  copper  melts  at  1081°,  and  the  admixture  of 
silver  lowers  its  freezing-point.  The  two  curves  in 
the  diagram  cut  each  other  at  a  point  which 
corresponds  with  a  temperature  of  777°,  and  a 
composition  of  40  atomic  percentages  of  silver  and 
60  of  copper.  At  other  points  on  the  curves,  the 
process  of  freezing  consists  in  the  separation  of 
primary  crystals  of  one  or  other  of  the  pure  metals 


10        20       30      4O       50       BO       7O 


Stiver 


Copper 


FIG.  6. 


in  the  manner  we  have  traced  for  solutions  in 
water.*  The  point  of  intersection  of  the  curves 
corresponds  to  the  point  of  saturation  both  of 
silver  with  copper  and  of  copper  with  silver. 

*  Osmond  thinks  that,  in  this  particular  case,  the  primary  crystals 
are  not  perfectly  pure.  He  adduces  evidence  to  show  that  a  slight 
trace  of  copper  is  dissolved  in  the  solid  crystals  of  silver.  Any  such 
effect,  however,  is  hardly  appreciable. 


8B  PHYSICAL  SCIENCE 

When  'the  fused  alloy  has  this  proportion,  crystals 
of  silver  and  copper  freeze  out  together,  just  as 
crystals  of  salt  and  water  freeze  out  together  when 
the  composition  of  the  solution  is  that  of  the 
cryohydrate.  The  point  we  are  considering,  then, 
corresponds  with  the  cryohydric  point  for  salt  and 
water.  The  composition  of  the  solid  is  here  the 
same  as  that  of  the  liquid,  and  therefore,  as  the 
process  of  solidification  goes  on,  the  residual  liquid 
always  has  a  constant  concentration.  Thus  the 
freezing-point  remains  constant  throughout  the 
operation,  and  is  identical  with  the  melting-point  at 
which  liquid  first  appears  when  the  solid  alloy  is 
heated.  Similar  phenomena  constantly  appear  in 
the  study  of  other  metals  ;  and  if  an  alloy  of  this 
composition  is  polished,  etched  with  acid,  and 
examined  under  a  microscope,  it  will  be  seen  to 
consist  of  a  uniform  conglomerate  of  the  two  kinds 
of  crystals,  whereas  an  alloy  of  any  other  propor- 
tion exhibits  larger  primary  crystals  of  that  metal 
which  is  present  in  excess,  and  was  frozen  out 
first,  connected  by  regions  filled  with  the  con- 
glomerate referred  to  above.  On  account  of  its 
more  uniform  texture  this  conglomerate,  which,  as 
we  have  seen,  corresponds  with  a  so-called  cryo- 
hydrate, is  named  the  etitectic  alloy.  Fig.  4, 
on  the  plate  facing  page  83,  represents  a  micro- 
scopic photograph  of  the  eutectic  of  gold  and  alu- 


FUSION  AND  SOLIDIFICATION          89 

minium  ;  while  in  Fig.  5  is  shown  the  structure 
of  an  alloy  with  a  composition  not  quite  that  of  the 
eutectic.  Here  large  primary  crystals  have  appeared, 
the  intervals  being  filled  with  the  same  eutectic 
which  is  seen  in  Fig.  4.  The  metal  of  Fig.  4 
has  been  cooled  more  slowly  than  that  of  Fig.  5, 
and  therefore  the  eutectic  in  Fig.  4  has  larger 
crystals  and  a  coarser  structure. 

The  eutectic  alloy  has  a  constant  melting  or 
freezing-point,  but,  during  the  process  of  fusion  or 
solidification  of  other  alloys,  the  temperature  will 
generally  change.  As  the  primary  crystals  of  one 
or  other  pure  metal  form,  they  leave  the  residual 
liquid  richer  in  the  other  constituent,  and  thus  with 
a  lower  freezing-point.  This  process  continues  till 
the  liquid  has  the  composition  of  the  eutectic  alloy, 
when  any  further  loss  of  heat  will  precipitate 
crystals  of  both  metals  side  by  side.  A  thermo- 
meter immersed  in  the  mixture  will  show  the 
temperature  at  which  primary  crystals  begin  to 
form,  and  the  temperature  at  which  the  composi- 
tion of  the  residual  liquid  reaches  that  of  the 
eutectic,  for  the  rate  at  which  it  falls  suddenly 
becomes  much  slower  when  solid  first  appears,  and 
the  fall  stops  altogether  while  the  eutectic  is 
freezing  out.  Thus,  in  such  a  simple  case  as  that 
of  silver  and  copper,  useful  information  can  be 
obtained  by  merely  drawing  the  curve  giving  the 


90  PHYSICAL  SCIENCE 

observed  relation  between  the  time  and  the  tem- 
perature for  the  heated  alloy.  Such  curves  have 
forms  more  or  less  resembling  that  shown  in 
Fig.  7. 

With  silver  and  copper  no  chemical  compounds 
are  formed  ;  with  many  pairs  of  metals  combina- 
tion occurs,  and  the  phenomena  are  more  com- 
plicated. A  definite  chemical  compound  plays  a 


Time/ 
FIG.  7. 

part  similar  to  that  of  a  pure  element.  Addition  of 
either  component  lowers  the  freezing-point  of  the 
compound.  Thus  the  point  of  solidification  of  the 
pure  compound  must  correspond  with  a  maximum 
point  on  the  equilibrium  curve.  If  a  single 
compound  is  formed  by  the  two  components,  the 
curve  must  consist  of  three  branches;  a  branch  due 
to  the  effect  of  the  compound  being  interposed 


FUSION   AND  SOLIDIFICATION          91 

between  two  branches  similar  to  those  in  the  silver- 
copper  curve  just  considered.  Copper  and  anti- 
mony form  a  single  compound  SbCu2,  in  which 
two  atoms  of  copper  are  united  to  one  of  anti- 
mony. The  equilibrium  of  the  solid  and  liquid 
phases  has  been  studied  by  M.  Le  Chatelier,  whose 
results  are  illustrated  in  Fig.  8.  In  this  case  two 


FIG.  8. 

eutectic  alloys  are  formed  ;  one  being  a  con- 
glomerate of  crystals  of  the  compound  with  those 
of  copper,  and  the  other  containing  crystals  of 
the  compound  and  crystals  of  antimony.  These 
eutectics  are  represented  by  the  points  a  and  c  in 
the  figure,  and  between  them  rises  the  curve 
showing  the  effect  of  the  compound,  which  exists 
in  the  pure  state  at  b,  the  maximum  of  the  curve. 


92  PHYSICAL  SCIENCE 

In  all  the  cases  yet  considered,  the  crystals 
deposited  consist  either  of  a  pure  metal  or  else  of  a 
pure  chemical  compound.  Whichever  it  be,  the 
composition  of  any  one  crystalline  species  is  fixed 
and  definite  ;  it  does  not  vary  continuously  when 
the  composition  of  the  mass  of  alloy  is  altered,  as 
does,  for  example,  the  composition  of  the  fused 
liquid.  In  Fig.  6,  page  87,  the  left-hand  branch 
of  the  curve  gives  the  composition  of  the  liquid 
alloy  which,  at  different  temperatures,  is  in 
equilibrium  with  crystals  of  pure  silver,  while  the 
right-hand  branch  represents  the  liquids  in 
equilibrium  with  pure  copper.  One  phase  only, 
the  liquid,  can  continuously  vary  in  composition  ; 
the  other,  or  solid,  phase  is  fixed  and  invariable. 
Similarly  in  the  case  illustrated  in  Fig.  8,  the 
crystals  of  the  compound  SbCu2  have  a  fixed  and 
constant  composition.  Cases  are  known,  however, 
in  which  the  solid  phase  also  varies  continuously. 
Many  salts,  such  as  the  different  alums,  are  of  the 
same  crystalline  form,  and  can  replace  each  other 
gradually  in  a  crystal,  which  may  have  any  com- 
position between  that  of  the  two  pure  salts. 
Such  structures  are  called  mixed  crystals  or  solid 
solutions.  When  they  can  exist,  the  phenomena 
of  equilibrium  become  much  more  complicated, 
for  the  composition  of  the  solid  will  vary  as 
well  as  that  of  the  liquid,  and  will  introduce 


\    '. 


J.  WILLARD  GIBBS 


To  face  page  93 


FUSION  AND  SOLIDIFICATION          93 

a  second  curve  into  the  freezing-point  dia- 
gram. 

It  is  only  of  recent  years  that  it  has  been  possible 
to  interpret  the  complicated  phenomena  of  solid 
solutions.  Now,  however,  we  possess  a  consistent 
theory  of  the  subject,  founded  by  Professor  Rooze- 
boom  of  Amsterdam,  on  the  work  of  the  late  Pro- 
fessor Willard  Gibbs  of  Yale  University.  Long 
ago,  in  the  years  1875  to  1878,  Gibbs  published  a 
series  of  mathematical  papers  in  the  Transactions 
of  the  Connecticut  Academy.  For  some  time  they 
remained  practically  unknown  to  European  physi- 
cists ;  then  they  were  discovered  by  Clerk  Maxwell, 
who  used  a  few  of  the  results  in  his  book  on  the 
"  Theory  of  Heat."  But  even  then  the  time  was 
not  ripe,  and  it  is  only  of  recent  years  that  we 
have  realised  that  the  whole  theory  of  chemical 
and  physical  equilibrium  is  contained  in  Gibbs' 
work.  Buried  for  so  long,  the  seed  has  germinated 
in  the  minds  of  many  investigators.  It  has  already 
borne  good  fruit,  and  is  probably  destined  to  bear 
still  more  in  time  to  come.  Happily,  Willard  Gibbs 
lived  to  see  a  general  recognition  of  his  genius, 
and  the  reputations  made  of  younger  men  who 
knew  how  to  extract  and  apply  even  single  results 
taken  from  the  rich  store  hidden  in  his  somewhat 
abstruse  pages. 

By  the  use  of  Gibbs'  thermodynamic  principles, 


94 


PHYSICAL  SCIENCE 


Roozeboom  was  able  to  trace  the  various  possible 
forms  which  could  be  assumed  by  the  two  curves, 
representing  the  compositions  of  the  liquid  and 
solid  phases  in  equilibrium  with  each  other.  The 
simplest  case  indicated  by  the  theory  is  shown 
in  Fig.  9.  In  regions  above  the  higher  curve, 
acb,  which  is  called  the  "liquidus,"  all  points 

represent  states  com- 
pletely liquid,  while  be- 
low the  curve  adb,  or 
"solidus,"  the  alloy  is 
entirely  solid.  Between 
these  curves  exist  both 
liquid  and  solid  in 
various  proportions.  At 
a  definite  temperature, 
a  liquid  of  one  com- 
position, say  c,  is  in 
equilibrium  with  a  solid 
of  another  composition, 
such  as  d.  As  the  process  of  solidification  pro- 
ceeds, the  composition  of  both  liquid  and  solid 
continuously  changes.  In  the  light  of  these  theo- 
retical curves,  the  complicated  experimental  curves, 
found  by  observing  the  freezing-points  of  mixtures 
of  metals  and  other  substances,  are  now  being 
rapidly  interpreted  in  a  manner  which  otherwise 
would  have  been  quite  impossible. 


Concervtrationi 
FIG.  9. 


FUSION  AND  SOLIDIFICATION          95 

One  of  the  most  successful  examples  of  such  an 
interpretation  is  given  by  the  very  thorough  study 
which  has  been  made  by  Heycock  and  Neville  of 
the  bronzes,  that  is,  of  alloys  consisting  of  copper 
and  tin.  The  curves  in  Fig.  10  show  the  results 
of  their  own  experiments  and  of  previous  work  by 
Roberts-Austen.  Heycock  and  Neville  examined 
microscopically  the  structure  of  various  alloys  of 
the  two  metals  in  conjunction  with  the  equilibrium 
curves,  and  have  given  us  a  knowledge  of  the 
bronzes  more  complete  than  we  possess  for  any 
other  series  of  alloys,  which  show  phenomena  of  an 
equal  degree  of  complexity. 

Fig.  10  (p.  96)  shows  the  equilibrium  curves,  from 
pure  copper  on  the  left  to  an  alloy  containing  80 
atomic  percentages  of  tin  on  the  right.  Above  the 
"  liquidus "  ABCDEFGH  the  alloys  consist  of  a 
homogeneous  liquid,  in  which  solid  first  begins  to 
form  when  the  temperature  falls  to  points  repre- 
sented on  the  curve.  The  "  solidus  "  curve,  below 
which  the  whole  mass  is  solid,  is  the  complicated 
curve  A^/rw</E2E4H//M. 

It  has  long  been  known  that  the  physical  pro- 
perties of  metals,  especially  of  alloys,  depend  on 
the  way  in  which  they  are  cooled  from  a  state  of 
fusion.  The  whole  process  of  the  annealing  or 
tempering  of  steel  depends  on  a  perception  of  this 
fact.  Many  observers  have  studied  the  changes  of 


96 


PHYSICAL  SCIENCE 


physical  properties  thus  produced  by  examining 
microscopically  the  solid  alloys  obtained  by  dif- 
ferent treatments,  and  relations  between  the  pro- 
perties of  the  alloy  and  its  microscopic  structure 
had  been  traced.  But  for  the  first  time  a  complete 
investigation  has  been  made  by  Heycock  and 

IVrceirtage   \>y  Weight    of    Tm 


Liquid 


300' 


OC+  (3 


/T*T 

£2                                                                             '  ^_ 

s+n 

_^ 

n  +  liq 

E3 

»)*H  +  Iiq.                          \ 
E,                                                %XxH*                          M 

r,  +  li  t   Tin 

FIG.  10. 

Neville  of  the  changes  in  microscopic  structure 
produced  by  different  methods  of  cooling,  and 
studied  in  conjunction  with  the  equilibrium  curves 
by  the  light  of  the  theory  of  solid  solutions.  The 
work  was  rendered  possible  by  the  fact  that,  if  a 
hot  metal  be  suddenly  cooled  from  any  tempera- 


FUSION  AND  SOLIDIFICATION         97 

ture  by  chilling  it  in  cold  water,  the  microscopic 
structure  it  possessed  at  that  temperature  is  almost 
perfectly  stereotyped  by  the  process  of  sudden 
chilling,  and  can  be  examined  at  leisure  in  the 
cold  metal  by  polishing  and  etching  it  in  the  usual 
manner. 

In  this  way  have  been  detected  and  traced 
equilibrium  curves  lying  below  the  solidus.  Such 
curves  represent  changes  of  structure  which  occur 
in  a  mass  completely  solid,  and  quite  explain  the 
changes  in  physical  properties  caused  by  annealing 
or  chilling.  Take  as  an  example  the  two  curves 
/x  and  E'X,  which  cut  each  other  in  the  point  x, 
and  recall  in  their  general  form  and  relations  the 
simple  curves  of  equilibrium  between  liquid  and 
solid  for  alloys  of  silver  and  copper  already  de- 
scribed and  illustrated  in  Fig.  6  (p.  87).  The  analogy 
is  more  than  one  of  mere  form.  Just  as  crystals 
of  silver  or  copper  separate  out  of  the  homogeneous 
liquid  of  Fig.  6,  so  crystals  of  new  substances 
separate  out  of  the  homogeneous  solid  solution 
which  exists  within  the  triangular  space  /x//  in 
Fig.  10 ;  and,  as  the  crystals  of  silver  or  copper 
are  in  equilibrium  with  the  liquid  alloy  in  states 
represented  by  points  on  the  freezing-point  curves 
of  Fig.  6,  so  the  new  crystalline  structures  are  in 
equilibrium  with  the  homogeneous  mother  sub- 
stance lying  within  our  present  triangle. 


98  PHYSICAL  SCIENCE 

The  positions  of  these  curves  of  equilibrium 
between  solid  phases  are  investigated  chiefly  by 
the  microscopic  examination  of  ingots  of  metal, 
which  are  fused,  allowed  to  cool  very  slowly  to  the 
temperature  to  be  investigated,  in  order  that,  as  far 
as  possible,  equilibrium  may  be  reached,  and  then 
suddenly  chilled  by  immersion  in  cold  water.  A 
section  of  the  ingot  is  polished  and  etched  with 
acid  or  other  suitable  liquid,  in  order  to  bring  out 
the  structure-pattern.  Each  pure  metal,  com- 
pound, or  solid  solution,  crystallising  from  the 
mother  liquid,  possesses  a  characteristic  appearance, 
which  can  readily  be  recognised  after  some  practice 
in  interpretation  of  the  micro-photographs.  Such 
photographs  enable  us  to  trace  the  formation,  de- 
velopment, and  decay  of  new  crystal-species  in  a 
liquid  or  in  a  solid  matrix. 

The  effect  on  the  microscopic  structure  of 
differences  in  the  rate  of  cooling  is  well  shown 
in  Figs,  n,  12,  and  13.  The  same  alloy  is 
represented  in  all  these  photographs,  and  is,  in 
each  case,  chilled  from  about  the  same  temperature. 
The  differences  in  structure  depend  solely  on  the 
differences  in  the  rate  of  cooling  from  a  liquid 
condition  to  the  temperature  at  which  the  ingot 
was  chilled  in  cold  water. 

The  alloy  contained  13.5  atomic  percentages  of 
tin,  and  is  represented  by  the  vertical  dotted  line 


FIG.  n. — Magnification  18 


FIG.  12. — Magnification  45 


FIG.  13.  — Magnification  18        FIG.  14. — Magnification  18       FIG.  15. — Magnification  18 


L  tof 


FIG.  16. — Magnification  18 


FIG  17. — Magnification  18 

To  face  page  99 


FUSION  AND  SOLIDIFICATION          99 

in  Fig.  9^  When  this  alloy  in  cooling  passes 
the  liquidus  ABC,  crystal  skeletons  of  a  solid 
solution  called  a  appear  mixed  with  the  mother 
liquid.  These  skeletons  somewhat  resemble  the 
larger  fern-like  structures  of  Fig.  2  on  page  83, 
which,  however,  chosen  chiefly  for  its  beauty,  was 
taken  from  a  bronze  of  another  composition. 

When  the  alloy  we  are  now  considering  passes 
the  line  /c  (Fig.  10),  a  new  kind  of  crystalline  solid 
solution,  called  ft,  begins  to  form  ;  and,  if  time  is 
given  it  by  keeping  the  ingot  hot,  the  /3  substance 
gradually  eats  up  the  existing  crystals  of  a.  This 
process  is  illustrated  in  Figs,  n,  12,  and  13.  In 
Fig.  1 1  the  residual  a  is  seen  as  white  cores  within 
the  grey  ft,  which  follows  the  arrangement  of  the 
original  a  structures,  while,  in  the  particular  illumi- 
nation employed,  the  part  that  was  liquid  at  the 
instant  of  chilling  shows  as  a  dark  background.  In 
Fig.  1 2,  where  the  ingot  was  cooled  more  slowly,  the 
change  has  gone  farther  ;  the  ft  substance  ceases  to 
follow  the  original  skeletons  of  a,  a  higher  mag- 
nification brings  out  the  characteristic  striated 
appearance  of  the  ft,  while,  owing  to  a  different 
illumination,  the  mother  liquid  shows  as  a  light 
background.  Fig.  13  is  taken  from  an  ingot 
which  had  been  cooled  to  the  same  chill  point 
exceedingly  slowly,  and  kept  many  hours  just 
above  that  temperature.  The  whole  ingot  is  now 


ioo  PHYSICAL  SCIENCE 

filled  with  uniform  striated  /3,  a  tiny  speck  of  a, 
seen  towards  the  lower  side  of  the  photograph,  alone 
remaining.  In  the  light  of  these  three  photographs 
it  is  not  surprising  that  the  physical  and  mechanical 
properties  of  metals  are  profoundly  modified  by 
differences  in  the  rates  at  which  they  have  been 
cooled  from  a  fused  condition. 

Following  the  dotted  line  in  Fig.^  still  further, 
we  see  that,  in  ingots  chilled  from  temperatures 
about  750°,  /8  alone  should  exist.  Fig.  14  shows 
a  chill  from  740°,  which  was  cooled  to  that 
temperature  almost  slowly  enough  to  destroy  all 
the  primary  crystals  of  a,  which  now  only  show  as 
scattered  specks  of  white. 

Again  following  the  dotted  line  in  the  equilibrium 
curve  of  Fig.  10,  we  pass  the  boundary  /x,  and 
again  enter  a  region  where  a  and  /3  exist  together. 
The  facts  on  which  this  curve  is  based  are  illus- 
trated in  Fig.  15.  Here  a  new  or  secondary  crop 
of  a  crystals  has  begun  to  grow.  This  ingot  was 
chilled  at  558°,  and  there  is  no  doubt  that  the 
new  growth  of  a  took  place  in  a  mass  which  had 
long  before  completely  solidified. 

The  further  growth  of  the  new  a  is  seen  in 
Fig.  1 6,  which  represents  an  alloy  of  slightly 
higher  content  of  tin  (14  atomic  per  cents)  chilled 
from  a  temperature  of  530°.  As  the  alloy  in 
cooling  passes  the  temperature  of  500°,  the  whole 


FUSION  AND   SOLIDIFICATION        101 

of  the  j8  substance  is  transformed  into  a  complex 
consisting  of  a  crystals  intimately  mixed  with  a  new 
solid  solution  called  S.  This  complex  is  shown 
in  Fig.  17  as  a  light  background  ;  while,  in  contrast 
with  it,  the  a  crystals  come  out  dark  after  the 
treatment  adopted. 

These  changes  again  occur  in  a  mass  thoroughly 
solid  throughout,  and  explain  in  a  most  striking 
manner  the  effect  of  such  processes  as  annealing 
and  tempering,  in  which  the  properties  of  metals 
are  altered  by  heating  them  to  a  temperature  well 
below  their  fusion  point  and  then  cooling  them 
either  slowly  or  rapidly. 

Heycock  and  Neville's  investigation  of  the 
bronzes  was  a  very  laborious  undertaking.  One 
hundred  micro-photographs  were  published,  and 
these  represent  only  a  selection  of  those  taken  ; 
many  observations  of  freezing-points  were  also 
made.  But  the  labour  of  the  work  is  well 
repaid  by  the  magnificent  results  finally  ob- 
tained. 

Iron  and  steel,  as  used  in  the  arts  and  indus- 
tries, consist  of  pure  iron  alloyed  with  various 
substances,  chiefly  carbon.  Solid  solutions,  similar 
to  those  we  have  studied  in  other  cases,  are  formed 
between  iron  and  carbon,  and  the  phenomena  of 
equilibrium  between  the  liquid  and  solid  phases, 


102  PHYSICAL  SCIENCE 

even  when  no  other  component  is  present,  are  very 
complicated. 

Owing  to  their  industrial  importance,  the  alloys 
of  iron  have  been  investigated  more  extensively 
than  those  of  any  other  metal,  and  the  various 
compounds  and  solid  solutions  identified  have 
received  definite  names,  which,  in  many  cases, 
were  given  long  before  the  application  by  Rooze- 
boom  of  the  theory  of  solid  solutions  enabled  the 
true  phenomena  of  equilibrium  to  be  understood. 
Roozeboom's  diagram  for  alloys  of  iron  and  carbon, 
containing  less  than  7  per  cent,  of  carbon,  is 
reproduced  in  Fig.  18.  Its  general  meaning  will 
be  clear  in  the  light  of  what  has  been  said  in  the 
case  of  the  bronzes.  Here  again  changes  occur 
at  definite  temperatures,  even  in  alloys  which  are 
completely  solid.  The  viscosity  of  the  material 
makes  these  changes  very  slow,  and  very  different 
proportions  of  the  various  possible  constituents 
will  be  found  in  alloys  that  have  been  cooled 
quickly  and  slowly.  The  effects  of  tempering 
steel  and  iron  thus  receive  a  physical  explana- 
tion. 

By  heating  iron  above  one  of  the  transformation 
temperatures  indicated  in  the  diagram,  and  main- 
taining it  at  a  high  temperature  for  some  time,  it 
will  obviously  be  possible  to  produce  extensive 
changes  in  the  physical  nature  of  the  metal. 


FUSION  AND  SOLIDIFICATION        103 

Recent  work  by  Mr.  J.  E.  Stead  has  shown,  that 
when  steel  rails  have  become  dangerously  brittle 
and  crystalline  by  long  use,  they  can  be  recon- 
verted into  a  tough,  elastic,  and  therefore  safe 


600     - 


condition  by  prolonged  heating  at  temperatures 
from  850°  to  900°  C.  This  improvement  in  pro- 
perties has  been  traced  to  the  development  of  a 
constituent  of  the  alloy  known  as  sorbite.  It  is 
this  constituent  which  gives  the  peculiar  tenacious 


104  PHYSICAL  SCIENCE 

properties  to  iron  which  has  been  specially  pre- 
pared for  drawing  into  wire. 

Microscopic  studies  of  the  alloys  composing  iron 
and  steel  have  been  very  numerous.  The  work  of 
Sorby,  Andrews,  Osmond,  Le  Chatelier,  and  Stead 
should  particularly  be  mentioned.  It  is  by  such 
microscopic  investigations  that  the  different  con- 
stituents of  the  alloys  have  been  for  the  most  part 
distinguished,  the  crystals  of  each  constituent 
having  a  characteristic  appearance,  which  usually 
persists  throughout  a  series  of  changes. 

The  investigations  we  have  described  all  em- 
phasise one  point — the  fact  that  metals  possess  a 
structure  essentially  crystalline.  In  some  cases, 
such  as  that  of  the  surfaces  of  zinc  deposited  on 
so-called  galvanised  iron,  this  crystalline  structure 
is  readily  visible,  but  most  of  the  metallic  objects 
in  common  use  possess  polished  surfaces  on  which 
no  trace  of  crystals  can  be  seen.  The  possibility 
of  polishing  a  surface  to  such  a  state  of  perfection 
that  it  will  act  as  a  mirror  and  reflect  a  ray  of  light 
without  appreciable  scattering,  is  a  matter  of  con- 
siderable interest.  Any  irregularities  on  such  a 
surface  must  be  small  compared  with  the  wave- 
lengths of  light,  and  it  is  difficult  to  see  how  any 
such  surface  could  be  obtained  by  the  use  of 
ordinary  polishing  materials,  if  the  action  of  these 


FIG.  19. — Magnification  775 


FIG.  20. — Magnification  775 


FIG.  21  —  Magnification  775 


FIG.  22. — Magnification  775 


FIG.  23. — Magnification  775 


FIG.  24. — Magnification  775 

To  face  •page  105 


FUSION  AND  SOLIDIFICATION        105 

materials  be  regarded  as  a  mere  mechanical 
grinding  away  of  projections  after  the  manner 
of  a  file. 

Many  careful  observations  have  been  made  on 
the  process  of  polishing.  The  latest  of  these 
observations  were  published  in  August  1903  in  the 
"  Proceedings  of  the  Royal  Society."  Mr.  G.  T. 
Beilby  has  investigated  the  subject  microscopically, 
and  finds  reason  to  believe  that  the  passage  over 
the  surface  of  a  scratched  metal  of  a  polishing 
substance  like  wash  leather  covered  with  rouge 
produces  a  kind  of  surface  flow,  the  outer  layers 
of  the  metal  flowing  like  a  viscous  liquid  under 
the  action  of  the  pressure  on  the  polishing  tool, 
and  assuming  an  optically  perfect  surface  under 
the  influence  of  surface  tension.  In  this  way  a 
film  is  formed  over  the  surface  of  a  metal,  which 
film  is  in  a  state  essentially  different  from  that  of 
the  bulk  of  the  substance  below.  Inside  the  metal 
the  crystalline  forces  have  full  play  ;  at  its  surface, 
the  controlling  influences  consist  in  part  of  surface 
tension,  which,  under  the  pressure  of  a  polishing 
tool,  is  able  to  overcome  the  tendency  to  assume 
a  crystalline  structure.  Figs.  19  to  24  show  six 
of  Mr.  Beilby's  photographs.  Fig.  19  shows  the 
surface  of  crystalline  antimony  after  rubbing  with 
fine  emery  paper.  The  magnification  is  such  that 
the  photograph  is  775  times  life-size.  Fig.  20, 


io6  PHYSICAL  SCIENCE 

which  represents  the  same  surface  after  polishing 
with  rouged  leather,  shows  the  gradual  dragging 
of  a  film  of  metal  over  the  pits  and  furrows  of  the 
first  surface.  The  larger  pits  get  filled  with  filings 
of  metal,  and  the  film  seems  to  bridge  them  over, 
forming  a  continuous  sheet  over  the  loosely-packed 
fragments  below.  When  an  acid  or  other  liquid 
capable  of  dissolving  the  metal  is  placed  on  the 
surface,  the  film  is  dissolved,  and  the  pits  and 
furrows  reappear.  This  comes  out  in  Fig.  21, 
in  which  the  antimony  previously  polished  has 
been  etched  with  a  solution  of  potassium  cyanide. 
Fig.  22  shows  a  polished  surface  of  speculum 
metal,  an  alloy  used  for  the  reflectors  of  tele- 
scopes. Here  the  underlying  crystalline  structure 
is  faintly  visible.  The  surface  film  has,  in  Fig. 
23,  been  removed  with  potassium  cyanide,  and 
the  structure  is  now  plain,  the  primary  crystals, 
separated  by  channels  of  eutectic  alloy,  being 
clearly  brought  out.  Finally,  in  Fig.  24,  the  same 
surface  has  been  repolished,  and  the  channels 
bridged  over  with  the  flowing  film  of  viscous 
metal. 

These  experiments  have  an  interest  which  ex- 
tends further  than  the  immediate  subject  to 
elucidate  which  they  were  undertaken — an  ex- 
perience not  uncommon  in  physical  research. 
The  existence  of  this  viscous  metallic  film  under 


FUSION  AND  SOLIDIFICATION        107 

certain  conditions  suggests  that,  when  minute 
quantities  of  a  solid  alone  exist — when  there  is  in 
effect  inside  the  surface  film  no  substance  beyond 
the  range  of  molecular  action — all  crystalline 
structure  must  disappear.  The  initial  formation 
of  solid  in  the  body  of  a  saturated  solution  or 
of  a  fused  material  will,  on  this  view,  be  exactly 
co-ordinated  with  the  deposition  of  drops  of  water 
from  a  mass  of  air  saturated  with  aqueous  vapour, 
and  the  possibility  of  super-saturation  will,  in 
each  case,  depend  on  the  work  required  to  form 
a  new  surface  of  separation  under  the  influence 
of  surface  tension  alone.  It  is  only  whejn  the 
individual  solid  structures  attain  a  considerable 
size  that  crystalline  forms  begin  to  appear. 


CHAPTER    IV 

THE    PROBLEMS    OF    SOLUTION 

"If  we  accept  the  hypothesis  that  the  elementary  substances  are 
composed  of  atoms,  we  cannot  avoid  concluding  that  electricity  also 
...  is  divided  into  definite  elementary  portions,  which  behave  like 
atoms  of  electricity." — H.  VON  HELMHOLTZ,  "Faraday  Lecture,"  1881. 

To  one  inexperienced  in  the  problems  which 
confront  the  workers  in  the  world  of  natural 
science,  the  whole  question  of  solution  and  its 
attendant  phenomena  may  appear,  at  first  sight, 
of  small  account.  Yet  the  study  of  these  same 
phenomena,  and  the  unravelling  of  their  intricate 
connections,  are  of  fundamental  importance.  Fur- 
thermore, as  the  work  of  the  last  twenty  years 
has  shown,  the  problems  involved  are  of  in- 
creasing interest,  not  only  from  the  point  of 
view  of  physics  and  chemistry,  but  also,  and 
perhaps  especially,  from  the  physiological  stand- 
point. More  and  more  the  reactions  of  inorganic 
substances,  whether  liquid  or  solid,  are  referred 
to  their  properties  in  a  state  of  solution,  while 
every  process  of  life  to  be  investigated  by  the 
biologist  seems  capable  of  interpretation  only 

108 


THE   PROBLEMS  OF  SOLUTION        109 

through  attention  to  the  conditions  thereby  in- 
volved. Moreover,  most  chemical  actions,  especi- 
ally those  easily  examined  in  the  laboratory,  occur 
between  substances  one  or  more  of  which  are 
actually  in  the  liquid  state  ;  while  the  application 
of  physical  conceptions  to  the  problems  of  living 
matter  chiefly  depends  on  the  knowledge  we 
possess  of  the  physics  and  chemistry  of  ordinary 
solutions. 

The  earliest  investigations  of  the  subject  were 
of  a  chemical  nature,  and,  till  the  passage  of 
electric  currents  through  liquids  came  to  be 
examined  at  the  beginning  of  the  nineteenth 
century,  little  systematic  study  of  the  physical 
properties  of  solutions  was  made.  But  since  that 
period  there  has  been  constant  progress,  and  many 
new  fields  of  research  have  been  opened  up. 

It  constantly  happens  that  light  is  thrown 
on  the  dark  places  of  one  science  by  work 
undertaken  to  elucidate  those  of  another;  and, 
in  this  case,  the  starting-point  for  the  modern 
theory  of  solution  is  found  in  some  experiments 
made  by  Pfeffer  in  1877  in  a  botanical  laboratory. 
Ten  years  earlier,  Traube,  in  studying  the  modes 
of  formation  of  the  organic  cells  of  plants  and 
animals,  had  discovered  how  to  construct  artificial 
membranes  permeable  to  water  but  not  to  solu- 


no  PHYSICAL  SCIENCE 

tions  of  certain  substances  dissolved  therein. 
Pfeffer  made  a  further  examination  of  these 
semi-permeable  membranes,  as  they  have  been 
called,  and  by  their  use  obtained  results  of  great 
importance  in  the  study  of  biology. 

A  porous  pot  of  unglazed  earthenware,  six  to 
eight  centimetres  high  and  two  or  three  centi- 
metres in  diameter,  is  sealed  by  means  of  sealing- 
wax  to  a  glass  tube,  as  shown  in  Fig.  25.  Having 
been  thoroughly  washed,  it  is  filled  with  the  solu- 
tion of  a  salt,  such  as  potassium  ferro-cyanide,  and 
the  outside  is  then  surrounded  with  the  solution 
of  another  salt,  such  as  copper  sulphate  or  ferric 
chloride,  which  gives  an  insoluble  precipitate 
when  in  contact  with  the  first  salt.  The  two 
solutions  gradually  diffuse  from  opposite  sides  into 
the  walls  of  the  cell,  and  form  an  insoluble  mem- 
brane where  they  meet  inside  the  thickness  of  the 
walls.  This  process  can  be  hastened,  and  the 
resulting  membrane  improved,  by  forcing  the 
salts  into  the  porous  material  by  means  of  an 
electric  current.  The  solutions  are  washed  away, 
and  the  wide  glass  tube  is  drawn  out  and  sealed 
to  a  smaller  tube  in  the  manner  shown  in  the 
figure. 

Inside  a  cell  thus  prepared  let  us  place  the 
solution  of  some  substance,  such  as  sugar  in  water, 
and  surround  the  outside  with  a  large  volume  of 


THE  PROBLEMS  OF  SOLUTION        in 
the  pure  solvent,  in  this  case,  water.     Water  will 


FIG.  25. 

gradually    force    its   way    into    the   cell,   and,   by 
placing  mercury  in  the  glass  tube  to  use  as  a  pres- 


H2  PHYSICAL  SCIENCE 

sure  gauge,  it  will  be  found  that  this  influx  will 
continue  till  a  definite  internal  pressure  is  reached. 
This  gives  a  measure  of  what  is  called  the  osmotic 
pressure  of  the  solution  as  it  finally  exists  in  the 
cell  after  the  entrance  of  the  additional  quantity 
of  water. 

Pfeffer  found  that  this  osmotic  pressure  was 
proportional  to  the  concentration  of  the  solution, 
at  all  events  between  the  concentrations  of  i 
to  6  per  cent,  of  sugar.  For  a  i  per  cent, 
solution,  the  pressure  at  6°. 8  C.  was  equal  to  that 
of  a  column  of  mercury  505  millimetres  high,  the 
normal  atmospheric  pressure  being  equivalent  to 
760  millimetres. 

Many  membranes  within  animal  and  vegetable 
organisms  are  semi-permeable,  or,  at  all  events, 
are  more  permeable  to  solvent  than  to  solution. 
The  permanent  or  temporary  differences  of 
pressure,  which  are  thus  set  up,  are  being 
extensively  investigated  by  physiologists,  and  have 
already  been  shown  to  play  important  parts  in 
the  processes  of  living  structures. 

Attention  was  first  called  to  the  interest  and 
importance  of  osmotic  pressure  from  a  physical 
standpoint  by  the  distinguished  Dutch  chemist, 
Van't  Hoff,  who  is  now  Professor  at  Berlin.  In 
1885  Van't  Hoff  pointed  out  that  Pfeffer's 
numbers  showed:  (i)  that  the  osmotic  pressure 


To  face  page  i 1 2 


THE   PROBLEMS  OF  SOLUTION        113 

was  inversely  proportional  to  the  volume  in  which 
a  given  mass  of  sugar  was  confined  ;  and  (2)  that 
the  absolute  value  of  the  pressure  in  the  case  of 
the  solution  of  sugar  was  the  same  as  that  which 
would  be  exerted  by  an  equal  number  of  mole- 
cules of  a  gas  when  placed  in  a  vessel  having 
a  volume  equal  to  that  of  the  solution.  For 
instance,  a  quantity  of  gas  of  the  same  molecular 
concentration  as  a  i  per  cent,  solution  of  sugar 
would,  at  6°. 8  C.,  exert  a  pressure  equivalent  to 
that  of  508  millimetres  of  mercury,  a  number 
identical  within  the  limits  of  experimental  error, 
with  Pfeffer's  observed  value  for  the  osmotic 
pressure.  The  first  result  is  equivalent  to  the 
extension  to  dilute  solutions  of  Boyle's  law  for 
gases,  a  law  which  states  the  experimental  result 
that  the  volume  of  a  gas  is  inversely  proportional 
to  its  pressure.  The  second  result  shows  that,  in 
a  dilute  solution,  the  pressure  depends  only  on  the 
number  of  molecules  present,  and  not  on  their 
nature — a  statement  which,  applied  to  gases,  is 
known  as  Avogadro's  law. 

But  Van't  Hoff  did  not  alone  call  attention  to 
the  experimental  basis  of  the  new  subject.  He 
also  placed  the  theory  of  it  on  a  sound  footing. 
The  amount  of  a  gas  which  dissolves  in  a  given 
quantity  of  water  is  proportional  to  the  pressure, 
and  from  this  experimental  result  Van't  Hoff 

H 


114  PHYSICAL  SCIENCE 

showed  mathematically  by  the  principles  of  thermo- 
dynamics, that,  when  in  solution,  this  same  gas 
must  exert  an  osmotic  pressure  of  the  observed 
value.  The  proof  involves  no  assumptions  as  to 
the  physical  mechanism  by  which  the  osmotic 
pressure  is  produced.  Whether  it  be  due  to  the 
impacts  of  the  dissolved  molecules  on  the  semi- 
permeable  walls,  in  the  same  way  that  the  mole- 
cules of  a  gas  exert  pressure  on  the  walls  of  the 
containing  vessel  ;  whether  it  be  due  to  chemical 
affinity  between  the  dissolved  substance  and  the 
solvent,  affinity  which  causes  more  solvent  to 
enter  the  cell ;  or  whether  some  other  hitherto 
untraced  effects  come  into  play,  remains  an  open 
question.  The  thermodynamic  argument  simply 
shows  that,  from  the  experimental  solubility  rela- 
tions of  gases,  the  observed  osmotic  results  follow 
for  the  gases  when  dissolved  ;  but  the  physical 
modus  operandi  of  the  pressure  remains  un- 
certain. 

The  extension  of  the  theoretical  result  to  the 
case  of  non-gaseous  solutes  like  sugar  involves  a 
certain  amount  of  assumption.  However,  since 
substances  of  all  degrees  of  volatility  are  known, 
the  extension  seems  reasonable  ;  and  it  is 
abundantly  justified  by  Pfeffer's  experimental 
measurements. 

Another  method  of  applying  the   principles  of 


t'f. 


THE  PROBLEMS  OF  SOLUTION        115 

thermodynamics  to  this  problem  has  been  de- 
veloped by  Willard  Gibbs,  Von  Helmholtz,  and 
Larmor.  Whatever  view  we  take  of  the  funda- 
mental nature  of  a  solution,  we  must  imagine  the 
dissolved  substance  scattered  as  a  number  of 
discrete  particles  throughout  the  volume  of  the 
solvent.  The  nature  of  the  interaction  which 
occurs  between  the  solute  and  the  solvent  is 
unknown,  possibly  unknowable  ;  but,  whatever  it 
may  be,  each  particle  of  solute  will  affect  only 
a  minute  sphere  of  solvent  lying  round  it.  The 
solution,  then,  may  be  regarded  as  containing  a 
number  of  little  systems,  each  composed  of  a 
solute  particle  surrounded  by  an  atmosphere  of 
solvent  in  some  way  influenced  by  its  nucleus. 

While  the  solution  is  concentrated,  the  little 
spheres  will  intersect  each  other,  and  the  addition 
of  further  solvent  will  involve  some  change  in  the 
interaction  between  solute  and  solvent.  But,  in 
the  process  of  dilution,  a  time  will  come  when  the 
spheres  are  beyond  each  other's  reach,  and  the 
addition  of  more  solvent  merely  increases  their 
mutual  separation  without  affecting  their  internal 
structure. 

Thus,  in  a  dilute  solution,  the  energy-change 
of  further  dilution  is  merely  the  energy-change 
involved  in  separating  the  particles  of  the  solute  ; 
it  will  not  depend  on  the  nature  of  any  possible 


u6  PHYSICAL  SCIENCE 

interaction  between  the  solute  and  the  solvent. 
The  change  of  energy  is  thus  independent  of 
the  nature  of  the  solvent,  and  will  be  the  same 
whether  that  solvent  be  water,  alcohol,  or  any 
other  liquid.  It  will  even  be  the  same  when, 
in  cases  where  it  is  possible,  the  solvent  is  re- 
moved altogether,  and  the  solute  is  obtained  in 
the  gaseous  state. 

If  we  imagine  that  the  bottom  of  a  frictionless 
engine  cylinder  is  made  of  a  semi-permeable  mem- 
brane, separating  a  solution  within  the  cylinder 
from  a  solvent  without,  it  is  easy  to  see  that 
osmotic  pressure  may  be  made  to  do  work,  which 
will  be  measured  by  the  pressure  multiplied  by  the 
change  of  volume.  Thus  the  osmotic  pressure  is 
measured  by  the  change  of  the  available  energy 
per  unit  increase  of  volume  ;  that  is,  by  the  rate 
of  change  in  the  available  energy  of  dilution. 

In  this  manner  we  again  arrive  at  the  con- 
clusion, that  the  osmotic  pressure  must  be  equal  in 
amount  to  the  gaseous  pressure  exerted  by  the 
same  number  of  molecules  when  vaporised,  and 
must  conform  to  the  laws  which  describe  the 
temperature,  pressure,  and  volume  relations  of 
gaseous  matter.  The  result  is  clearly  seen  to  be 
independent  of  any  hypothesis  concerning  the 
mechanism  of  the  pressure  or  the  nature  of  the 
solution. 


THE   PROBLEMS  OF  SOLUTION        117 

In  the  last  chapter  we  have  traced  the 
phenomena  of  fusion  and  solidification,  and,  in  the 
course  of  our  inquiry,  studied  the  equilibrium  of 
liquid  solutions  with  the  different  solid  phases 
which  may  exist  in  contact  with  the  liquids.  The 
fundamental  problem  of  the  nature  of  a  solution 
was  untouched  ;  indeed,  from  the  point  of  view 
then  adopted,  such  a  problem  did  not  arise. 

Until  the  last  quarter  of  the  nineteenth  century, 
it  was  generally  assumed  that  the  forces  which 
were  brought  into  play  when  a  solid  dissolved  in 
water  were  of  the  same  nature  as  those  involved  in 
chemical  action  ;  and  the  resulting  solution  was 
looked  on  simply  as  a  chemical  compound  in 
which  there  happened  to  be  no  fixed  relation 
between  the  masses  of  the  components.  The  study 
of  dilute  solutions,  and,  in  particular,  the  examina- 
tion of  their  osmotic  pressures,  showed  that,  in 
many  respects,  a  dilute  solution  was  analogous  to 
a  gas,  and  conformed  with  the  same  laws  relating 
to  the  pressure,  volume,  and  temperature.  Such 
results  emphasised  the  analogy  between  the  dissolu- 
tion of  a  solid  and  the  diffusion  of  a  gas  through 
a  space  in  which  it  was  not  originally  present,  and 
sometimes  led  to  the  idea  that  the  osmotic  pressure 
of  a  solution,  like  the  pressure  of  a  gas,  was  due 
to  the  impact  of  its  molecules  on  the  containing 
wall.  As  an  extreme  case  of  this  aspect  of  the 


n8  PHYSICAL  SCIENCE 

phenomena,  the  view  has  been  expressed  that  the 
solvent  should  simply  be  regarded  as  giving  room 
for  the  diffusion  of  the  molecules  of  the  solid  ; 
any  possible  interaction,  of  a  chemical  nature  or 
otherwise,  between  the  solvent  and  solute  being 
disregarded. 

The  similarity  between  the  laws  of  gases  and 
those  of  dilute  solutions,  however,  does  not  neces- 
sarily connote  identity  in  physical  nature  ;  the 
account  of  the  subject  given  by  thermodynamics 
clearly  shows  that  the  essential  feature,  common 
to  both  cases,  on  which  the  similarity  depends,  is 
the  dilution.  In  a  gas  the  molecules  are,  on  the 
average,  too  far  from  each  other  to  exert  appreciable 
intermolecular  forces,  and  the  change  in  energy 
produced  by  further  dilution  does  not  involve  such 
intermolecular  forces.  In  the  same  way  the 
dissolved  molecules  in  a  dilute  solution  are  so  far 
from  each  other  that,  whatever  be  their  action  on 
the  solvent,  they  exert  none  on  each  other.  Here 
again,  the  change  of  energy  on  further  dilution 
does  not  involve  the  forces  between  those  molecules 
which  alone  from  this  point  of  view  are  to  be 
considered,  -that  is,  the  molecules  of  the  dissolved 
substance.  The  essential  point  is  the  distant 
separation  of  the  molecules  in  each  case  from  each 
other  ;  any  interaction  between  solvent  and  solute 
would  not  affect  the  result,  and  the  result  therefore 


THE  PROBLEMS  OF  SOLUTION        119 

cannot  be  used  as  evidence    for  or  against  such 
interaction. 

The  similarity  in  pressure-volume  laws,  then, 
cannot  be  regarded  as  determining  the  question 
whether  solution  is,  in  its  essential  nature, 
chemical  or  physical.  To  settle  such  a  problem 
other  evidence  must  be  sought.  Very  little  such 
evidence  is  yet  available  ;  what  little  there  is 
seems  rather  to  favour  the  chemical  view,  which 
regards  a  solution,  say  of  salt  and  water,  as  in 
some  way  a  chemical  compound  of  these  com- 
ponents ;  a  compound  in  which  the  relative 
proportion  between  the  components  can  vary 
continuously  between  certain  wide  limits. 

The  results  in  this  case  are  characteristic  of  the 
methods  of  thermodynamic  theory  as  applied  in 
physical  science.  Thermodynamics  is  not  con- 
cerned with  the  physical  modus  operandi  of  the 
phenomena.  It  does  not  involve  molecular  hypo- 
theses ;  it  is  free  from  any  doubt  which  ac- 
companies such  hypotheses,  though  it  gives  less 
insight  into  the  intimate  processes  of  the  pheno- 
mena than  do  successful  molecular  conceptions. 

In  the  development  of  several  branches  of 
physics  and  chemistry  two  stages  can  be  traced. 
It  has  sometimes  happened  that  the  earliest 
theoretical  account  of  a  subject  has  been  given 
from  the  mechanical  or  molecular  standpoint.  In 


120  PHYSICAL  SCIENCE 

this  way  a  definite  working  hypothesis  has  arisen, 
on  the  lines  of  which  much  investigation  has  been 
undertaken.  Gradually,  however,  this  preliminary 
scaffolding  has  been  found  to  be  unnecessary,  and 
a  thermodynamic  theory  has  been  developed,  which 
directly  connects  the  phenomena,  and  brings  out 
their  relations  with  similar  phenomena  in  other 
branches  of  science. 

The  two  methods  may  perhaps  be  illustrated  in 
some  such  way  as  the  following.  In  looking  at 
the  face  of  a  watch,  certain  relations  are  observed 
between  the  positions  Of  the  two  hands  at  different 
times.  In  order  to  explain  these  phenomena  we 
make  hypotheses  concerning  the  structure  of  the 
inside  of  the  watch.  We  imagine  various  arrange- 
ments of  springs,  wheels,  and  levers  till  we  hit  on 
one  particular  system  which  consideration  shows 
us  will  give  the  observed  result.  Here  we  have  an 
intimate  picture  of  the  inside  of  the  watch,  which 
may  or  may  not  represent  the  only  possible 
arrangement,  and  may  or  may  not  correspond 
with  the  reality.  Such  a  picture  is  analogous  to 
a  molecular  theory  of  a  physical  problem. 

One  day,  however,  we  notice,  in  the  course  of 
our  studies  of  the  watch,  that,  whatever  be  the 
position  of  the  hands,  one  of  them  always  moves 
sixt}^  times  as  fast  as  the  other.  We  have 
discovered  a  necessary  relation  between  the 


THE   PROBLEMS  OF  SOLUTION        121 

phenomena,  which  enables  us,  if  we  will,  to 
dispense  with  all  hypotheses  about  the  wheels  and 
springs  which  drive  the  mechanism.  The  observed 
connection  between  the  rates  of  motion  allows  us 
to  evade  all  such  complications,  and  to  calculate 
directly  the  relative  positions  of  the  two  hands  at 
any  future  time. 

So  with  thermodynamics.  Lord  Kelvin's  great 
principle  of  the  dissipation  of  energy,  especially  in 
its  modern  form,  which  states  that  the  available 
energy  of  an  isothermal  system  tends  constantly  to 
decrease,  enables  us  in  many  cases  to  evade  all 
molecular  considerations,  and  to  trace  directly  the 
relations  between  various  physical  and  chemical 
phenomena.  By  this  method  it  is  possible  to 
develop  the  theoretical  relations  of  many  subjects 
without  involving  the  molecular  hypothesis.  Such 
treatment,  using  as  its  sole  principle  of  co-ordina- 
tion the  law  of  available  energy,  ultimately  rests  on 
the  experimental  impossibility  of  perpetual  motion. 

This  way  of  treating  physical  science  has 
recently  been  adopted  by  a  certain  number  of 
chemists,  as  a  means  of  presenting  their  subject 
without  applying  to  it  the  language  or  conceptions 
of  the  atomic  theory,  in  terms  of  which  even  its 
simplest  experimental  facts  have  come  to  be  ex- 
pressed. In  particular  Franz  Wald  and  Ostwald 
have  explained  the  phenomena  of  chemical  com- 


122  PHYSICAL  SCIENCE 

bination  in  definite  proportions  from  the  stand- 
point of  energetics.  They  have  shown  that  the 
existence  of  the  two  types  known  to  us  as  elements 
and  compounds  may  be  deduced  from  the  thermo- 
dynamic  theory  of  equilibrium  without  reference 
to  atomic  hypotheses.  But,  in  the  present  state 
of  knowledge,  such  a  doctrine  seems  limited  in 
its  scope,  and  cases  in  which  it  ceases  to  be 
sufficient  will  constantly  recur  in  this  volume. 
For  instance,  the  phenomena  of  highly  rarified 
gases  have  only  been  interpreted  successfully  by 
the  aid  of  strictly  molecular  conceptions.  The 
passage  of  electricity  through  gases,  which  will  be 
considered  in  a  future  chapter,  leads  directly  to 
molecular  hypotheses,  and,  in  conjunction  with 
the  phenomena  of  radio-activity,  gives  an  extended 
insight  into  the  intimate  structure  of  atoms  and 
molecules.  In  such  matters  we  are  driven  back 
to  molecular  theory,  which  offers  an  alternative 
method  of  correlating  other  phenomena  also, 
equally  definite,  if  in  some  ways  more  speculative. 

Thermodynamic  theory,  as  well  as  practical 
experiment,  thus  indicates  that  the  osmotic  pres- 
sure of  a  solution  depends  only  on  the  number  of 
dissolved  particles,  and  not  on  their  nature  or  on 
the  nature  of  the  solvent.  The  phenomena  of 
gases  show  that  the  number  of  molecules  in  two 


THE  PROBLEMS  OF  SOLUTION        123 

systems  may  be  compared  by  a  knowledge  of 
the  total  masses  and  of  the  chemical  molecular 
weights.  Thus,  two  solutions,  one  of  sugar,  let 
us  suppose,  and  one  of  alcohol,  which  are  prepared 
so  as  to  contain  the  same  number  of  molecules  in 
the  same  volume,  both  in  theory  and  practice, 
possess  equal  osmotic  pressures.  But,  if  equi- 
molecular  solutions  of  sugar  and  salt  be  examined, 
the  osmotic  pressure  of  the  salt  is  found  to  be 
greater,  and,  if  the  solutions  be  dilute,  nearly  twice 
as  great  as  that  of  the  sugar.  These  abnormally 
great  osmotic  pressures  were  discovered  at  an  early 
date  in  the  history  of  the  subject ;  and  further  in- 
vestigation showed  that,  at  all  events  when  the  sol- 
vent was  water,  they  occurred  in  the  cases  of  those 
solutions  which  were  conductors  of  electricity. 

When  Van't  Hoff  formulated  the  physical  theory 
of  the  osmotic  pressure,  he  treated  these  abnormal 
values  as  exceptions  to  the  usual  law.  It  was 
reserved  for  the  physicists  Arrhenius  of  Stockholm 
and  Planck  of  Berlin  to  point  out  that  the  exten- 
sion of  Van't  Hoff's  principles  to  these  cases 
required  the  assumption  of  the  dissociation  of  the 
molecules  of  salt  in  order  that  the  total  number  of 
particles  in  solution  should  still  be  the  number 
indicated  by  the  observed  phenomena.  According 
to  this  hypothesis,  in  a  dilute  solution  of  common 
salt,  the  solute  does  not  exist  as  molecules  of 


124  PHYSICAL  SCIENCE 

sodium  chloride,  but  as  the  dissociated  parts, 
sodium  and  chlorine,  which,  since  the  solution 
conducts  a  current  of  electricity,  must  be  asso- 
ciated with  electric  charges.  Each  salt  molecule 
thus  gives  two  pressure-producing  particles  in 
solution,  and  the  double  value  of  the  osmotic 
pressure  is  explained.  In  stronger  solutions  this 
dissociation  is  not  complete,  and  the  osmotic 
pressure  is  less  than  twice  the  normal  value  ;  but 
no  exact  correlation  of  pressure  and  dissociation 
can  be  made,  for  the  thermodynamic  theory  as 
formulated  above  is  only  valid  for  very  dilute 
solutions. 

Like  the  thermodynamic  theory  of  osmotic 
pressure  generally,  this  extension  of  it  does  not 
involve  any  particular  view  as  to  the  cause  of 
the  pressure  or  the  nature  of  solution.  The 
dissociation  hypothesis  is  concerned  simply  with 
the  difference  between  solutions  of  electrolytes 
and  non-electrolytes,  and  leaves  entirely  open 
the  more  fundamental  question,  whether  solution 
is  essentially  chemical  or  physical  in  its  nature. 

The  dissociation  theory  of  aqueous  solutions 
of  electrolytes,  originally  indicated  by  osmotic 
phenomena,  is  supported  perhaps  even  more 
clearly  and  strongly,  by  the  study  of  the  elec- 
trical properties.  During  the  years  1830  to  1840, 


THE  PROBLEMS  OF  SOLUTION        125 

Faraday  made  a  series  of  experiments  on  the 
passage  of  electricity  through  liquids,  in  this  way 
laying  the  foundations  of  our  quantitative  know- 
ledge of  that  subject.  He  showed  that  the 
transfer  of  a  given  quantity  of  electricity  was 
always  accompanied  by  the  liberation  of  a  definite 
quantity  of  one  of  the  constituents  of  the  solution, 
a  quantity  proportional  to  the  total  electric  trans- 
fer, and  to  the  chemical  equivalent  weight  of  the 
substance  liberated.  The  quantity  of  electricity 
which  passed,  then,  depended  on  the  number  of 
chemical  equivalents  of  substance  liberated  and 
not  on  their  nature.  These  results  led  to  a 
definite  view  as  to  the  nature  of  the  process  of 
electrolysis.  We  must  regard  the  passage  of  an 
electric  current  through  a  solution  as  due  to  the 
carriage  by  moving  parts  of  the  salt  of  opposite 
electric  charges  in  opposite  directions  through 
the  liquid.  Under  the  influence  of  applied  electric 
forces,  these  carriers  drift  through  the  solu- 
tion, and  finally  give  up  their  charges  to  the 
electrodes,  as  the  terminals  by  which  the  current 
enters  and  leaves  the  solution  are  called.  With 
common  salt,  for  example,  a  stream  of  positively 
electrified  sodium  drifts  with  the  electric  cur- 
rent, while  negatively  electrified  chlorine  passes 
in  the  opposite  direction.  The  moving  parts  of 
the  salt,  with  their  accompanying  electric  charges, 


126  PHYSICAL  SCIENCE 

were  named  ions  by  Faraday  ;  the  positive  ion 
which  moves  down  the  electric  current,  is  termed 
the  cation,  and  the  negative  ion  which  travels  up 
the  electric  stream  is  called  the  anion.  The 
electrodes  to  which  they  travel  are  known  as  the 
cathode  and  anode  respectively.  The  electric 
charge  on  a  single  ion  of  a  substance  like  sodium 
or  chlorine  constitutes  a  true  natural  unit  of 
electricity.  No  smaller  quantity  seems  capable 
of  existing.  As  Helmholtz  has  insisted,  electricity, 
like  matter,  is  not  infinitely  divisible  ;  it  possesses 
an  atomic  structure. 

In  the  year  1855  Hittorf  examined  the  changes 
in  the  concentration  of  a  solution  which  occur  on 
the  passage  of  an  electric  current,  and  explained 
them  by  supposing  that  the  two  ions  moved  at 
unequal  rates.  It  is  evident  that  more  salt  will  be 
taken  from  that  end  of  the  solution  from  which 
comes  the  more  mobile  ion,  and,  on  the  assump- 
tion that  this  is  the  only  cause  at  work,  Hittorf 
calculated  the  ratio  between  the  velocities  of  the 
two  ions  in  many  cases. 

The  next  great  step  was  made  by  Kohlrausch, 
in  1873.  The  conductivity  of  a  solution  is 
measured  by  the  total  quantity  of  electricity 
which  passes  through  the  solution  per  second 
under  the  action  of  a  given  electric  force  ;  and, 
since  the  current  is  carried  by  the  motion  of 


THE   PROBLEMS  OF  SOLUTION        127 

charged  ions,  the  conductivity  must  depend  on  the 
number  of  the  ions,  that  is,  on  the  concentration 
of  the  solution,  and  on  the  velocity  with  which  the 
opposite  ions  move  through  the  liquid.  Thus, 
by  measuring  the  conductivity,  the  velocities  of 
the  ions  under  a  given  electric  force  can  be 
calculated. 

So  far  the  movement  of  the  ions  was  visible 
to  the  mind's  eye  only.  Their  passage  through 
a  solution  seemed  necessary  to  explain  the  facts, 
and,  in  an  indirect  way,  their  velocities  could  be 
calculated,  but  no  direct  evidence  of  the  reality 
of  these  hypothetical  phenomena  was  forthcoming. 
However,  in  the  year  1886  Sir  Oliver  Lodge, 
and  shortly  afterwards  by  a  somewhat  different 
method  the  present  writer,  showed  how  to 
render  these  molecular  processes  visible,  and  how 
to  watch  the  motion  of  the  ions  as  they  drift 
through  the  solution  under  the  action  of  the 
electric  forces. 

One  apparatus  which  may  be  used  for  this 
purpose  is  represented  in  Fig.  26.  Let  us  suppose 
that  a  solution  of  some  coloured  salt  is  placed 
in  contact  with  the  solution  of  some  colourless 
one,  so  that  a  fairly  sharp  line  of  demarca- 
tion is  produced  between  them.  The  solutions 
should  be  of  the  same  molecular  concentration, 
the  same  conductivity,  and  the  denser  solution 


128 


PHYSICAL  SCIENCE 


must,  of  course,  be  placed  below  the  lighter. 
Let  us  take,  as  an  example,  the  case  of  solutions 
of  potassium  bichromate  and  potassium  carbonate, 
which  fulfil  the  necessary 
conditions.  The  colour  is 
here  due  to  the  acid  part  of 
the  salt,  the  bichromate  ion, 
which  has  the  chemical 
composition  represented  by 
Cr2O7;  the  potassium  ion  is 
colourless.  When  a  current 
of  electricity  is  passed  across 
the  junction  between  the 
liquids,  the  colour  boundary 
is  seen  to  move,  and,  from 
the  rate  at  which  it  creeps 
along  the  tube,  the  velocity 
of  the  bichromate  ion  under 
a  given  electric  force  can  be 
determined. 

The  conductivity  of  a  salt 
solution,  made  solid  by  the 
addition  of  gelatine  or  some 
similar  substance,  is  nearly 
the  same  as  that  of  the  liquid 

solution  without  the  jelly,  and  this  fact  justifies 
the  use  of  such  solid  solutions  in  experiments 
on  the  migration  of  ions.  Lodge  determined  the 


FIG.  26. 


THE   PROBLEMS  OF  SOLUTION        129 

velocity  of  the  hydrogen  ion  by  watching  the  rate 
at  which,  passing  along  a  glass  tube,  it  changed 
the  colour  of  an  indicator,  while  the  present  writer 
has  measured  the  velocity  of  many  other  ions  by 
tracing  the  formation  of  opaque  precipitates,  formed 
in  minute  quantity  by  the  ions  in  their  path. 

Of  late  years  these  methods  have  been  im- 
proved and  extended  by  Orme-Masson  and  B.  D. 
Steele.  The  general  result  of  the  experiments  is 
to  confirm  the  values  for  the  ionic  velocities  calcu- 
lated from  the  theories  of  Kohlrausch  and  Hittorf. 

The  velocities  with  which  the  ions  travel,  even 
when  driven  forward  by  intense  electric  forces, 
are  very  small.  Hydrogen,  the  most  mobile  ion 
known,  moves  over  a  distance  of  ten  centimetres, 
or  four  inches,  in  one  hour,  when  the  applied 
electromotive  force  is  one  volt  per  centimetre. 
Most  other  ions  travel  at  about  one-tenth  this  rate. 

These  comparatively  small  velocities  must  not  be 
confounded  with  an  entirely  different  thing :  the 
velocity  with  which  an  electric  impulse,  started  at 
one  end  of  a  tube  filled  with  an  electrolyte,  reaches 
the  other  end.  This  velocity  is  very  great,  closely 
approaching  the  rate  at  which  an  electro-magnetic 
wave  travels  through  free  space,  that  is,  the  velo- 
city of  light,  about  one  hundred  and  eighty 

thousand  miles  a  second. 

I 


130  PHYSICAL  SCIENCE 

If  we  accept  for  the  moment  the  common  con- 
ception of  an  electric  current  as  analogous  to  the 
flow  of  a  liquid  through  a  conducting  pipe,  the 
connection  between  the  two  modes  of  motion  may 
be  illustrated  by  a  familiar  example.  Suppose 
that  a  long  wooden  rod  is  lying  on  the  surface  of 
the  ground,  and  that  a  push  is  given  to  one  end  of 
it.  The  motion  of  the  rod  may  be  quite  slow,  an 
inch  an  hour  if  we  like.  But,  after  moving  one 
end,  the  other  end  begins  to  move  an  extremely 
minute  fraction  of  a  second  after  the  starting  of 
the  impulse.  Perhaps  it  never  has  occurred  to 
us  that  any  appreciable  time  elapses  between  the 
starting  of  the  two  ends.  Yet,  if  we  think  for  a 
moment,  it  is  clear  that  the  initial  push  must  travel 
as  a  wave  of  compression  along  the  rod,  and  that 
the  far  end  can  only  begin  to  move  when  the  wave 
front  reaches  it.  The  bearing  of  the  analogy  is 
now  obvious.  The  slow  movement  of  the  rod  as 
a  whole  when  once  started  corresponds  with  the 
slow  drift  of  the  ions  ;  the  almost  instantaneous 
passage  of  the  wave  of  compression  along  the  rod 
corresponds  with  the  velocity  of  electricity  in  the 
electrolytic  solution. 

A  picture  of  the  phenomena,  more  nearly  cor- 
responding with  the  facts,  is  obtained  by  considering 
that  the  rapid  electric  impulse  travels  as  an  electric 
wave  through  the  surrounding  luminiferous  aether. 


THE   PROBLEMS  OF  SOLUTION        131 

On  this  view,  due  to  Faraday  and  Maxwell, 
and  now  universally  accepted,  the  electric  forces 
always  travel  through  the  aether.  When  they  act 
on  charged  matter  free  to  move,  as  in  metallic 
conductors  or  electrolytic  solutions,  they  produce 
a  drift  of  that  matter — a  drift  which  constitutes  a 
current.  Along  the  line  of  the  drift,  that  is,  along  a 
conductor,  energy  is  lost,  and  thus  along  that  line, 
and  there  alone,  energy  is  constantly  flowing,  being 
carried  forward  by  the  aether  to  supply  the  place 
of  the  energy  dissipated  by  the  current. 

The  mobility  of  any  one  ion  is,  in  dilute  solu- 
tions, independent  of  the  nature  of  the  other  ion 
present,  at  all  events  in  simple  salts,  such  as  the 
chlorides  of  sodium,  potassium,  and  lithium.  This 
independence  itself  indicates  that  the  ions  are 
independent  of  each  other,  and  again  suggests 
some  form  of  dissociation. 

The  phenomena  of  conductivity  also  point  to 
the  same  idea.  To  set  free  an  ion  or  its  products 
at  the  electrodes  requires  the  expenditure  of  a 
certain  amount  of  electric  work,  and  at  the  elec- 
trodes an  equivalent  reverse  electro-motive  force 
exists.  When,  however,  this  reverse  force  is  over- 
come, the  passage  of  the  current  through  the 
solution  is  opposed  by  no  other  reversible  forces, 
and  it  is  found  that  the  work  expended  is  that 


132  PHYSICAL  SCIENCE 

required  to  force  the  current  against  the  resistance 
of  the  electrolyte  alone.  The  current  is  propor- 
tional to  the  excess  of  the  electric  force  applied 
beyond  what  is  needed  to  overcome  the  effect  at 
the  electrodes ;  this  part  of  the  conduction  con- 
forms with  Ohm's  law,  which  describes  the  pro- 
cess in  metallic  conductors,  In  the  body  of  the 
solution,  then,  as  distinct  from  the  transition 
layer  in  contact  with  the  electrodes,  the 
electric  forces  do  no  reversible  work,  such  as 
would  be  needed  to  separate  the  ions  from  each 
other.  Whatever  freedom  is  requisite  between 
the  ions  for  the  purpose  of  conduction,  must 
necessarily  exist  whether  the  electric  forces 
act  or  not  ;  the  function  of  the  electric  forces 
when  applied  is  simply  to  force  the  ions,  already 
separated  from  each  other,  against  the  frictional 
resistance  of  the  liquid  medium.  A  certain  free- 
dom of  interchange,  at  all  events,  is  thus  indicated 
between  the  ions,  and  the  freedom  of  interchange 
exists  whether  the  current  passes  or  not.  Such 
freedom,  indeed,  had  long  ago  been  inferred  from 
the  phenomena  of  double  decomposition  observed 
in  the  chemical  reactions  between  solutions  of 
different  salts. 

So  far  the  conductivity  relations  indicate  the 
possibility  of  ionic  interchange  between  the  parts  of 
the  dissolved  molecules,  though  the  conformity  of 


THE  PROBLEMS  OF  SOLUTION        133 

solutions  with  Ohm's  law  does  not,  of  itself,  neces- 
sitate the  idea  of  permanent  ionic  freedom.  But 
on  any  other  view  the  possibility  of  interchange 
must  be  secured  by  collisions  between  the  dis- 
solved molecules,  and  consequent  interchanges 
between  their  ions,  which  would  thus  work  their 
way  through  the  solution  by  a  series  of  such  col- 
lisions. The  velocity  with  which  this  process  was 
effected  must  then  depend  on  the  frequency  of 
collision,  which  would  be  proportional  to  the 
square  of  the  concentration.  The  ionic  velocities, 
then,  on  this  supposition,  would  increase  in  pro- 
portion to  the  square  of  the  concentration  of  the 
solution,  and  the  conductivity,  which  depends  on 
the  product  of  the  ionic  velocities  and  the  concen- 
tration, would  vary  as  the  cube  or  third  power 
of  the  concentration. 

But  the  facts  are  quite  inconsistent  with  this 
hypothesis.  The  conductivity  is  proportional  at 
the  most  to  the  first  power  of  the  concentration  ; 
and  the  ionic  velocities,  instead  of  increasing  as 
the  square,  are,  in  dilute  solution,  independent  of 
the  concentration,  and  in  more  concentrated  solu- 
tions decrease  with  increasing  concentration.  We 
are  thus  again  driven  to  the  belief  that  the  ions 
are  free  from  each  other,  and  move  independently 
of  each  other  through  the  liquid  under  an  electric 
force :  free  from  union  with  each  other,  let  us 


134  PHYSICAL  SCIENCE 

observe,  not  necessarily  free  from  combination, 
chemical  or  other,  with  the  solvent.  As  already 
indicated,  the  dissociation  theory  does  not  depend 
on  any  particular  view  as  to  the  nature  of  solution 
in  general. 

For  aqueous  solutions,  then,  the  evidence  in 
favour  of  the  dissociation  hypothesis  is  very  strong, 
and  it  can  safely  be  used  as  a  working  hypothesis 
to  co-ordinate  the  known  phenomena,  and  to  guide 
future  research.  For  solutions  in  other  solvents, 
less  evidence  is  yet  available ;  though  for  solutions 
of  certain  salts  in  alcohol,  the  laws  of  the  elec- 
trolysis seem  to  be  similar  to  those  of  aqueous 
solutions  and  to  indicate  a  similar  theory.  In 
fused  salts,  and  solid  electrolytes  like  the  filaments 
of  Nernst  lamps,  which  also  conduct  currents  of 
electricity,  the  conditions  are  different,  and  we 
must  wait  for  further  light  before  we  can  profit- 
ably examine  the  nature  of  the  conduction  process. 

Besides  explaining  the  electrical  and  osmotic 
properties  of  solutions,  the  dissociation  theory  has 
proved  one  of  the  most  fruitful  generalisations  in 
the  domain  of  chemistry  that  have  ever  been 
formulated.  Solutions  of  salts  and  acids,  electro- 
lytes in  fact,  are  the  solutions  which  exhibit 
chemical  activity  in  the  highest  degree.  In 
them,  the  ions  alone  are  concerned  in  chemical 


THE  PROBLEMS  OF  SOLUTION        135 

action,  and  so  clearly  is  this  the  case,  that, 
as  soon  as  the  subject  is  examined,  the  ordi- 
nary chemical  tests  for  the  presence  of  salts 
are  seen  at  once  to  be,  in  reality,  tests  for 
the  individual  ions  of  those  salts.  At  one 
time  it  seemed  likely  that  all  cases  of  rapid 
chemical  action  might  be  reduced  to  reactions 
between  electrolytic  ions,  but  recent  work  by 
Kahlenberg  and  others  seems  to  show  that  in  non- 
aqueous  solvents  rapid  reactions  may  occur  not 
in  any  way  correlated  with  electrolytic  conduc- 
tivity. However  this  may  be,  in  water  many 
chemical  actions  are  certainly  connected  in  a  very 
intimate  way  with  the  electrical  properties,  and 
the  dissociation  theory  gives  a  satisfactory  method 
of  co-ordinating  the  two  sets  of  properties.  In 
some  reactions  the  actual  electric  charges  on  the 
ions  seem  to  be  the  determining  factors  of  the 
whole  process. 

There  is  a  marked  difference  in  chemical  and 
physical  properties  between  bodies  of  definite 
crystalline  form,  such  as  most  inorganic  salts, 
and  soft  or  amorphous  substances,  such  as 
albumen  and  the  various  kinds  of  jelly.  Long 
ago  Graham  distinguished  the  two  groups  as 
crystalloids  and  colloids  respectively,  and  par- 
ticularly examined  them  with  regard  to  their 


136  PHYSICAL  SCIENCE 

relative  powers  of  diffusion  through  water.  He 
found  that,  while  crystalloids  diffuse  comparatively 
rapidly,  the  motion  of  colloids  is  so  slow  that 
it  is  often  almost  inappreciable. 

Many  different  kinds  of  chemical  compounds 
show  colloidal  properties.  Besides  a  vast  number 
of  animal  and  vegetable  substances,  some  of 
which  seem  to  play  a  great  part  in  the  pheno- 
mena distinctive  of  living  matter,  many  of  the 
precipitates  which  are  formed  in  the  course  of 
inorganic  chemical  reactions  appear  in  an  amor- 
phous or  colloidal  state.  The  sulphides  of  such 
metals  as  antimony  and  arsenic  are  good  ex- 
amples. If  a  solution  of  arsenious  acid  is  allowed 
to  flow  into  water  kept  saturated  with  sulphuretted 
hydrogen  by  means  of  a  current  of  that  gas,  a 
colloidal  hydrosulphide  is  formed.  Many  hydrates, 
too,  are  colloids,  ferric  hydrate,  for  instance,  which 
can  readily  be  prepared  from  the  corresponding 
salts  of  iron.  By  treating  dilute  solutions  of 
gold  chloride  with  reducing  agents,  such  as  a 
few  drops  of  a  solution  of  phosphorus  in 
ether,  the  gold  is  set  free  in  the  colloidal  con- 
dition, forming  a  ruby-coloured  solution.  Silver, 
bismuth,  and  mercury  can  also  be  obtained  in 
colloidal  solution. 

Crystalloids  diffuse  much  more  rapidly  through 
water  and  other  solvents  than  do  colloids.  If 


THE  PROBLEMS  OF  SOLUTION        137 

a  mixture  of  crystalloids  and  colloids  be  placed 
in  a  drum  covered  with  a  colloidal  membrane, 
such  as  bladder  or  parchment,  complete  separa- 
tion can  be  effected,  for  the  dissolved  colloids 
seem  quite  incapable  of  passing  through  such 
membranes.  This  process  probably  plays  a  great 
part  in  animal  and  vegetable  physiology. 

Solutions  of  colloids  in  crystalloid  solvents,  such 
as  water  or  alcohol,  seem  to  be  divisible  into  two 
classes.  Both  classes  appear  to  mix  with  warm 
water  in  all  proportions,  and  the  mass  will  solidify 
under  certain  conditions  to  form  a  solid  which 
may  be  called  a  gel.  One  class,  represented  by 
gelatine  and  agar  jelly,  will,  when  solidified,  re- 
dissolve  on  warming  or  dilution,  while  the  other 
class,  containing  such  substances  as  hydrated 
silica,  albumen,  aud  metallic  hydro-sulphides,  will, 
under  the  influence  of  heat  or  on  the  addition 
of  electrolytes,  form  gels  which  cannot  be  re- 
dissolved.  The  solidification  of  members  of  the 
first  class  into  redissolvable  substances  is  termed 
setting,  that  of  substances  in  the  second  class, 
which  form  insoluble  precipitates,  is  termed 
coagulation. 

The  mechanism  of  gelation  in  the  first,  or 
reversible  class  of  colloidal  systems,  has  been 
studied  experimentally  by  Van  Bemmelen  and 
by  W.  B.  Hardy.  The  process  of  solidification 


138  PHYSICAL  SCIENCE 

seems  to  consist  in  the  growth  of  a  solid  frame- 
work containing  more  liquid  portions.  The  tem- 
perature at  which  this  separation  into  two  phases 
occurs  depends  on  the  amount  of  water  present. 

The  coagulation  of  irreversible  colloidal  solu- 
tions; as  already  stated,  can  be  effected  by  the 
addition  of  small  quantities  of  the  solution  of  an 
electrolyte,  such  as  an  ordinary  salt  or  acid. 
Graham,  who  originally  investigated  the  subject, 
found  that  a  minute  trace  of  salt  was  often 
sufficient.  Thus,  hydrated  alumina,  prepared 
from  a  solution  of  the  chloride,  was  so  unstable 
that  a  few  drops  of  well-water  at  once  produced 
coagulation,  and  the  same  change  was  brought 
about  by  pouring  the  colloidal  solution  into  a 
new  glass  vessel,  unless  the  vessel  had  previously 
been  washed  repeatedly  with  distilled  water. 

Several  experimenters,  including  Schulze,  Lin- 
der  and  Picton,  and  Hardy,  have  recently  worked 
at  this  coagulative  power  of  electrolytes,  with 
very  curious  and  interesting  results.  The  coagu- 
lative power  of  a  salt  is  found  to  vary  in  a 
remarkable  manner  with  the  chemical  valency 
of  one  of  its  ions.1  The  average  of  the  coagu- 

1  The  valency  of  a  chemical  atom  may  be  defined  as  the  number 
of  hydrogen  atoms  it  will  combine  with  or  replace.  Thus  the  normal 
valency  of  oxygen  is  two,  since  two  hydrogen  atoms  unite  with  one 
oxygen  atom  to  form  water:  Faraday's  work  showed  that  the  electric 
charge  carried  by  an  ion  is  proportional  to  its  valency. 


THE  PROBLEMS  OF  SOLUTION        139 

lative  powers  of  salts  of  univalent,  divalent,  and 
trivalent  metals  are  found  to  be  proportional 
to  the  numbers  i  :  35  :  1023  respectively. 
Most  properties  which  depend  on  the  valency 
vary  in  the  ratios  1:2:3,  anc*  the  great  differ- 
ence in  the  numbers  now  under  consideration 
is  very  striking.  An  explanation  of  these  unusual 
relations  has  been  given  by  the  present  writer. 

Let  us  now  frame  a  mental  picture  of  a  solution 
as  it  is  represented  by  the  dissociation  theory. 
A  certain  number  of  the  dissolved  molecules 
are  regarded  as  dissociated  into  charged  ions, 
which  wander,  free  from  each  other,  through 
the  liquid,  perhaps  by  successive  combinations 
with  solvent  molecules  in  their  path.  When  an 
electric  force  is  applied,  though  still  moving 
sometimes  in  one  direction  and  sometimes  in 
another,  the  ions,  on  the  whole,  drift  in  the 
direction  indicated  by  the  force,  and  we  may 
therefore  imagine  that  two  processions  of  oppo- 
sitely charged  ions  pass  each  other,  drifting  in 
opposite  directions  through  the  solution. 

When  there  is  no  electric  force,  the  ions  are  sub- 
ject to  no  steady  drift,  and  must  move  sometimes 
in  one  direction,  sometimes  in  another,  as  the 
chances  of  their  life  direct.  Any  one  ion  will 
sometimes  be  passing  from  one  solvent  molecule 
to  another  carrying  its  electric  charge  with  it; 


140  PHYSICAL  SCIENCE 

sometimes  it  will  come  across  an  ion  of  the 
opposite  kind  in  such  a  way  that  combination 
occurs,  and,  for  a  time,  an  electrically  neutral 
molecule  is  formed.  By  collisions  of  unusual 
violence,  or  by  other  means,  this  molecule  will  soon 
be  dissociated,  and  its  ions  again  set  free  from 
each  other,  to  be  handed  backwards  and  forwards 
by  the  solvent  molecules  as  already  described. 

Let  us  suppose  that,  in  order  to  produce  the 
aggregation  of  colloidal  particles  which  constitute 
coagulation,  a  certain  minimum  electric  charge 
has  to  be  brought  within  reach  of  a  colloidal 
group,  and  that  such  conjunctions  must  occur 
with  a  certain  minimum  frequency  throughout 
the  solution.  Since  the  electric  charge  on  an 
ion  is  proportional  to  its  valency,  we  shall  get 
equal  charges  by  the  conjunction  of  in  triads, 
3#  diads,  or  6n  monads,  where  n  is  any  whole 
number. 

The  chance  conjunctions  of  a  large  number 
of  particles  moving  like  the  ions  of  an  electrolytic 
solution  can  be  investigated  by  the  principles  of 
the  kinetic  theory  of  gases.  If  i/x  denote  the 
chance  of  one  ion  colliding  with  a  colloidal 
particle,  the  chance  that  two  ions  should  collide 
with  it  is  the  product  of  their  separate  chances, 
or  i/x2,  and  so  on.  When  applied  to  the 
case  in  hand,  these  principles  lead  to  the 


THE  PROBLEMS  OF  SOLUTION        141 

conclusion  that  the  relative  coagulative  powers 
of  univalent,  divalent,  and  trivalent  ions  will  be 
proportional  to  the  ratios  i  :  n  :  nz.  The  value 
of  n,  which  depends  on  a  number  of  unknown 
factors,  remains  arbitrary.  If  we  assume  that 
n  is  32,  n2  is  1024,  and  we  get  the  numbers 
i  :  32  :  1024  to  compare  with  the  experi- 
mental values  of  the  relative  coagulative  powers 
i  :  35  :  1023. 

When  we  consider  the  difficulty  of  the  experi- 
ments, and  remember  that  the  coagulative  powers 
of  different  solutions  containing  ions  of  equal 
valency  are  not  exactly  equal,  but  vary  as  the 
equivalent  conductivities  of  the  solutions,  we  see 
that  these  results  show  a  remarkable  agreement 
with  the  calculated  numbers,  and  give  strong 
evidence  in  favour  of  the  hypothesis  that  coagu- 
lation depends  on  the  presence  of  a  minimum 
electric  charge,  which  is  brought  into  action  by 
the  chance  conjunction  of  the  ions  of  an  electrolyte. 

The  particles  in  solutions  of  colloids  in  water 
generally  move  slowly  when  acted  on  by  electric 
forces,  the  direction  of  motion  depending  on  the 
nature  of  the  colloid  and  on  that  of  the  solvent. 
Hardy  found  that  the  direction  of  movement  of 
certain  proteids  could  be  changed  by  changing  the 
solvent  from  a  very  dilute  acid  to  a  very  dilute 
alkali.  This  reversal  implied  a  change  in  the  sign  of 


142  PHYSICAL  SCIENCE 

the  charges  on  the  colloid  particles  ;  and,  if  the  sol- 
vent was  very  carefully  neutralised,  an  iso-electric 
point  was  reached  at  which  the  solution  became 
very  unstable,  and  coagulation  seemed  to  occur 
spontaneously.  The  same  observer  also  found 
that,  in  the  case  of  colloids  travelling  with  the 
current,  it  is  the  acid  ion  which  is  active  in 
causing  coagulation,  and  not  the  metallic  ion  as  in 
the  experiments  of  the  older  experimenters,  who 
all  used  colloids  which  travel  against  the  electric 
current.  Thus  it  is  always  the  ion  possessing  a 
charge  of  opposite  kind  to  that  on  the  colloid 
particle  which  is  effective  in  producing  coagulation. 

These  results  are  of  great  importance,  not  only 
from  the  point  of  view  of  physiology,  from  which 
they  were  undertaken,  but  also  as  throwing  light 
on  the  nature  of  colloid  solution,  perhaps,  indeed, 
of  solution  in  general.  It  looks  as  though  colloid 
particles,  at  any  rate,  could  exist  in  solution  only 
when  charged  electrically.  If,  by  the  conjunction 
of  more  mobile  ions,  their  charge  is  neutralised  and 
the  iso-electric  point  reached,  coagulation  must  im- 
mediately follow. 

It  is  probable  that  these  effects  depend  on 
changes  in  the  surface  of  separation  between 
the  colloidal  particles  and  the  more  liquid 
phase  which  surrounds  them.  Such  a  surface 
of  separation  must  exhibit  the  well-known 


THE  PROBLEMS  OF  SOLUTION        143 

phenomena  of  surface-tension,  and  will  possess 
an  amount  of  available  energy  proportional  to 
its  area,  which  therefore  tends  to  become  as 
small  as  possible.  A  number  of  separate  par- 
ticles would,  in  these  conditions,  tend  to 
coagulate  into  larger  ones,  just  as  small  rain- 
drops tend  to  coalesce  into  larger  ones.  If 
the  colloidal  particles  are  electrified,  the  electric 
energy  is  greater  when  the  charge  is  concen- 
trated on  a  small  area,  and,  on  this  account, 
the  area  will  tend  to  increase.  The  effect  of 
the  electric  charge  is  thus  opposite  to  that  of 
the  natural  surface-tension,  and  diminishes  the 
tendency  to  coagulate.  Thus  an  electric  charge 
may  enable  the  colloid  to  dissolve,  while  neutrali- 
sation of  the  charge  may  result  in  coagulation. 

Modern  physiology  finds  some  reason  for 
believing  that  a  wave  of  this  electrolytic  coagu- 
lation is  the  physical  accompaniment  of  a 
nerve  impulse,  while  permanent  and  irreversible 
coagulation  results  from  the  action  of  certain 
poisons.  This,  however,  is  not  the  place  to  follow 
in  detail  such  an  interesting  inquiry,  which  deals 
with  matters  outside  the  present  scope  of  physical 
science. 

Much  discussion  has  taken  place  about  the 
nature  of  liquid  colloidal  solutions,  and  their 
relations  with  ordinary  solutions  of  mineral  salts 


144  PHYSICAL  SCIENCE 

and  other  crystalloids.  They  may  either  be 
regarded  as  ordinary  solutions;  in  which  the 
dissolved  particles  are  similar  in  kind  to  those  of 
crystalloid  solutions,  though  of  much  higher  mole- 
cular weight,  or  they  may  be  considered  to  be 
systems  of  two  phases,  composed  of  suspensions 
of  particles  in  the  liquid,  different  from  it,  and  of 
much  greater  than  molecular  dimensions. 

In  some  colloid  solutions  the  presence  of 
suspended  particles  can  readily  be  detected  by 
ordinary  means.  Sometimes  they  may  be  visible 
under  a  good  microscope  ;  in  other  cases,  while 
too  small  to  be  directly  visible,  they  are  large 
enough  to  scatter  and  polarise  a  beam  of  light. 
This  means  that  their  size  must  be  comparable 
with  the  wave-length  of  light,  about  5  x  io~5  C. 
Such  particles  would  be  too  few  in  number 
to  exert  a  measurable  osmotic  pressure,  and  the 
absence  of  such  pressure  does  not  necessarily 
mean  that  solutions  of  colloids  are  different  in 
kind  from  solutions  of  crystalloids. 

It  is  worthy  of  note  that  turbid  suspensions  of 
clay,  kaoline,  &c.,  in  water  are  rapidly  cleared  by 
the  addition  of  small  quantities  of  metallic  salts. 
This  action,  which  is  almost  certainly  of  the  same 
nature  as  the  coagulation  described  above,  pro- 
bably helps  in  the  formation  of  sand-banks  at  the 
mouths  of  rivers ;  the  salts  of  the  sea-water  clear 


THE   PROBLEMS  OF  SOLUTION        145 

the  suspensions  of  clay  brought  down  with  the 
fresh  water,  and  precipitation  is  then  aided  by 
the  diminished  velocity. 

The  conditions  which  determine  the  colloid  or 
crystalloid  nature  of  a  substance  are  still  not  fully 
understood.  The  persistence  of  colloid  properties, 
when  a  substance  passes  from  the  dissolved  to  the 
non-dissolved  state,  shows  that  the  determining 
conditions  must  be  of  fundamental  importance. 
The  molecular  forces  seem  to  be  much  less  active 
in  colloids,  but  the  freedom  with  which  some  of 
them  disintegrate  and  dissolve  in  presence  of  water 
and  other  liquids  indicates  that  some  interaction 
between  them  and  their  solvent  must  occur.  It 
seems  likely  that  the  forces  which  are  involved  in 
crystalloid  solution  are  of  the  nature  of  those 
classed  as  chemical  or  molecular,  while,  when 
colloids  dissolve,  the  actions  between  solvent  and 
solute  are  conditioned  also  by  the  phenomena 
studied  under  the  names  of  capillarity  and  surface 
tension.  It  is  not  likely  that  any  sharp  line  of 
demarcation  can  be  drawn  ;  though,  as  the  size 
of  the  dissolved  particles  increases,  the  importance 
of  the  chemical  forces  probably  diminishes,  and 
that  of  the  capillary  forces  grows. 

If  colloid  and  crystalloid  solution  are  but  the 
extreme  limits  of  a  continuous  series  of  phenomena, 
the  study  of  dissolved  colloids  of  varying  degrees 

K 


146  PHYSICAL  SCIENCE 

of  aggregation  should  throw  much  light  on  the 
general  problem  of  the  fundamental  nature  of 
solution. 

The  explanation  of  the  coagulation  of  colloidal 
solutions  as  an  effect  on  the  surface  conditions  at 
the  junction  between  colloid  and  solvent,  brought 
about  by  the  chance  conjunctions  of  dissociated 
electric  ions,  is  an  illustration  of  a  course  of 
history  which  indeed  constantly  repeats  itself  in 
scientific  inquiry.  An  observation  is  made,  per- 
haps long  series  of  experiments  are  carried  out, 
before  the  general  state  of  knowledge  enables  a 
satisfactory  explanation  of  the  phenomena  to 
be  formed,  or  a  theoretical  co-ordination  of 
them  with  other  phenomena  to  be  traced.  Even 
Graham's  acute  and  powerful  mind,  in  the  absence 
of  the  dissociation  theory  of  electrolytes,  and  of 
the  knowledge  of  the  surface  relations  of  two 
phases  which  we  now  possess,  could  frame  no 
complete  theory  of  the  coagulation  effects  which 
he  examined  with  such  skill.  By  experiments  on 
coagulation  alone  it  is  probable  than  an  explana- 
tion could  never  have  been  reached.  But  by 
the  advance  of  other  observers,  led  by  Gibbs 
on  one  far-off  flank,  and  by  Van't  Hoff  and 
Arrhenius  on  the  other,  almost  out  of  touch  with 
the  original  attack,  the  position  of  the  adversary 


THE  PROBLEMS  OF  SOLUTION        147 

— ignorance — was  turned  ;  and  when,  at  a  later 
time,  a  new  frontal  assault  was  made,  the  way 
proved  easy  and  obvious. 

"  For,  while  the  tired  waves,  vainly  breaking, 

Seem  here  no  painful  inch  to  gain, 
Far  back,  through  creeks  and  inlets  making, 
Comes,  silent,  flooding  in,  the  main. 

And  not  by  eastern  windows  only, 
When  daylight  comes,  comes  in  the  light ; 

In  front  the  Sun  climbs  slow,  how  slowly  ! 
But  westward,  look  !  the  land  is  bright." 


CHAPTER    V 

THE    CONDUCTION    OF    ELECTRICITY    THROUGH 
GASES 

"It  is  difficult  to  think  of  a  single  branch  of  the  physical  sciences 
in  which  these  advances  are  not  of  fundamental  importance.  .  .  . 
The  physicist  sees  the  relations  between  electricity  and  matter  laid 
bare  in  a  manner  hardly  hoped  for  hitherto.  .  .  .  But  it  is  the 
philosopher  that  these  researches  will  affect  most  profoundly.  As 
much  by  the  aid  of  a  perfect  mastery  over  the  properties  of  materials 
as  by  the  sheer  intellectual  power  of  abstract  reasoning,  some 
of  the  fundamental  problems  of  the  constitution  of  matter  are  here 
presented  as  on  the  verge  of  solution." — Times,  January  22,  1904. 

UNLIKE  the  liquid  solutions  and  other  electrolytes 
studied  in  the  last  chapter,  gases,  in  normal 
conditions,  are  almost  perfect  insulators  of  elec- 
tricity. Telegraph  wires  are  insulated  by  the  air 
which  surrounds  them,  and,  if  leakage  occurs  to 
any  measurable  extent,  it  can  always  be  traced  to 
the  solid  supports  to  which  the  wires  are  attached. 
Nevertheless,  by  delicate  instruments,  a  slight 
leakage  of  electricity  through  air  can  be  detected. 
This  air  leakage  is  usually  extremely  small,  but  it 
can  be  increased  greatly  in  many  ways.  The 
passage  of  Rontgen  rays,  the  incidence  of  ultra- 
violet light  on  a  metal  plate,  the  neighbourhood 


To  face  page  148 


CONDUCTION  THROUGH  GASES       149 

of  flames,  incandescent  metals,  or  of  radio-active 
bodies  such  as  radium,  are  among  the  ionizing 
agencies  whereby  the  condition  of  the  surround- 
ing air  is  modified  so  that  it  can  rapidly  conduct 
away  the  electric  charge. 

In  general,  the  currents  through  gases  are  too 
small  to  be  investigated  by  means  of  a  galvano- 
meter. By  the  aid  of  an  electrometer,  however, 
or  by  the  use  of  some  form  of  gold  leaf  electro- 
scope, the  passage  of  electricity  may  be  detected, 
and  the  amount  of  the  current  determined. 

The  quadrant  electrometer  consists  of  a  light 
needle  of  aluminium  or  silvered  paper,  suspended 
by  a  fine  quartz  fibre.  The  needle  is  kept 
charged  with  electricity,  and  is  therefore  deflected 
when  other  charges  are  given  to  brass  quadrants 
which  surround  it.  By  the  rate  at  which  the 
deflection  of  the  needle  diminishes,  it  is  possible 
to  estimate  the  rate  at  which  the  charge  on  the 
quadrants,  and  on  any  conductor  connected  with 
them,  disappears  or  increases. 

Still  simpler  and  yet  more  sensitive  is  the  gold 
leaf  electroscope,  in  which  a  thin  strip  of  gold  leaf 
is  attached  to  a  brass  plate,  and  charged  with 
electricity.  Owing  to  the  repulsive  forces  between 
portions  of  the  same  charge,  the  gold  leaf  is 
repelled  from  the  plate  and  stands  out  at  an 
angle.  By  observing  through  a  microscope  the 


150  PHYSICAL  SCIENCE 

rate  at  which  the  leaf  falls,  we  can  determine  the 
rate  at  which  its  charge  leaks  away. 

Whichever  apparatus  is  adopted,  the  natural 
leak,  due  to  the  apparatus  itself  and  the  air  sur- 
rounding it,  must  first  be  determined,  and  sub- 
tracted from  the  leakage  afterwards  found  under 
the  influence  of  an  ionizing  agency. 

In  the  last  chapter  we  have  seen  that  the 
properties  of  conducting  solutions  have  been 
successfully  co-ordinated  and  explained  on  the 
hypothesis  that  the  passage  of  a  current  is  effected 
by  the  motion  of  charged  particles  called  ions.  A 
similar  supposition  has  been  adopted  to  explain 
the  conductivity  of  gases,  although  it  will  be  clear 
that,  in  many  respects,  the  ions  in  the  case  of 
electric  discharge  through  gases  must  be  endowed 
with  properties  different  from  those  which  pertain 
to  the  ions  of  liquid  solutions. 

After  a  period  of  activity  on  the  part  of  some 
ionizing  agency,  such  as  Rontgen  rays,  the  resultant 
conductivity  does  not  cease  simultaneously  with  the 
action  of  the  rays.  It  persists  for  some  little  time ; 
it  can  be  blown  about  with  currents  of  air  ;  and 
in  all  respects  acts  as  though  it  were  due  to  the 
presence  of  material  particles,  formed  somehow  in 
the  gas  through  which  the  rays  had  passed.  The 
conductivity  is  destroyed  if  the  gas  be  passed 
through  a  plug  of  glass  wool  or  bubbled  through 


CONDUCTION  THROUGH  GASES       151 

water  ;  it  is  also  removed  if  the  gas  be  subjected 
to  the  action  of  an  electric  field.  Such  experi- 
ments, and  many  others  of  somewhat  similar 
nature,  are  readily  explained  by  the  conception  of 
charged  particles,  which,  produced  in  some  way 
by  the  action  of  the  ionizing  agency  on  the 
molecules  of  the  gas,  are  afterwards  driven 
through  the  gas  by  an  electric  force,  just  as  the 
ions  of  a  salt  solution  are  driven  through  the  liquid. 
Unlike  the  ions  of  liquids,  however,  those  of  gases 
do  not  long  persist  after  the  cessation  of  the 
outside  ionizing  agency.  Left  to  themselves,  the 
ions  gradually  disappear.  Such  a  disappearance 
might  be  anticipated  on  the  view  that  the  opposite 
ions  re-combine  and  neutralize  each  other,  and 
also  on  the  assumption  that  they  give  up  their 
charges  to  the  solid  objects  with  which  they  come 
in  contact  as  they  move  about  under  their  own 
motions  of  diffusion,  and  that  they  are  driven 
towards  an  electrode  by  the  action  of  an  electric 
force. 

The  non  -  persistence  of  gaseous  ions  and  the 
consequent  need  of  their  perpetual  renewal  ex- 
plains the  relation  between  current  and  electro- 
motive force  —  a  relation  different  from  that 
observed  in  liquid  solutions.  In  solutions,  as  we 
saw,  the  conduction  conforms  with  Ohm's  law — the 
current  is  proportional  to  the  electro-motive  force. 


152  PHYSICAL  SCIENCE 

In  gases  this  is  not  the  case.  For  an  ionizing 
agency  of  constant  intensity,  such  as  a  layer  of 
oxide  of  uranium,  the  current  at  first  rises  with 
the  applied  electro-motive  force,  but  it  soon  tends 
towards  a  limit,  and  finally  reaches  a  maximum, 
when,  till  we  approach  the  sparking  point,  no 
further  increase  of  electro-motive  force  will 


Electromotive,   Force 
FIG.  27. 

produce  any  appreciable  increase  of  current. 
This  saturation  current,  as  it  is  called,  is  repre- 
sented by  the  horizontal  part  of  the  curve  in 
Fig.  27.  Obviously  it  corresponds  to  a  state 
in  which  all  the  ions  are  removed  to  the 
electrodes  as  fast  as  they  are  produced  by  the 
ionizing  agency. 


CONDUCTION  THROUGH  GASES      153 

As  the  sparking  point  is  approached,  the  curve 
shows  that  the  current  again  rises  rapidly  ;  the 
applied  electric  force  being  strong  enough  to  pro- 
duce ions  in  the  gas  by  its  own  action.  Townsend 
has  shown  that  this  process  is  effected  by  the 
collision  with  the  gas  molecules  of  ions  already 
present,  which  are  driven  forward  by  the  electric 
force  with  high  velocity.  In  this  way  are  formed 
most  of  the  ions  which  carry  the  current  in 
an  electric  spark,  or  in  the  arc  discharge. 

We  have  already  described  the  methods  of 
calculating  the  velocities  with  which  the  ions  of 
liquids  move  under  known  electric  forces,  and  of 
determining  those  velocities  by  direct  experiment. 
For  gaseous  ions,  the  corresponding  velocities  are 
much  higher.  They  have  been  determined  in 
several  indirect  ways,  with  concordant  results. 
For  instance,  Zeleny  measured  the  electric  force 
required  to  push  an  ion  against  a  stream  of  gas, 
moving  with  a  known  and  uniform  velocity  in  the 
opposite  direction  to  the  natural  motion  of  the  ion. 
Langevin,  in  1902,  attacked  the  problem  in  another 
way.  The  gas  between  two  parallel  electrodes  was 
exposed  momentarily  to  the  action  of  Rontgen 
rays.  The  ions  thus  produced  may  disappear  in 
two  ways.  Opposite  ions  may  re-combine  with 
each  other,  or  they  may  pass  to  the  electrodes 
under  the  influence  of  an  electric  force.  If  the 


154  PHYSICAL  SCIENCE 

force  be  great,  the  latter  method  alone  is  operative, 
the  number  of  ions  re-combining  before  reaching 
the  electrodes  being  very  small.  If,  then,  the 
electric  field  be  kept  acting  in  one  direction,  all 
the  positive  ions  produced  by  the  Rontgen  rays 
will  go  to  one  electrode,  and  all  the  negative  ions 
to  the  other.  But  if  the  electric  force  be  reversed 
before  all  the  ions  get  across,  the  charge  received 
by  an  electrode  would  be  less  than  before.  Thus, 
measurement  of  the  charges  received  by  the  elec- 
trodes with  different  speeds  of  reversal  will  give  a 
means  of  calculating  the  velocities  of  the  ions. 
At  atmospheric  pressure,  under  a  potential  gradient 
of  one  volt  per  centimetre,  the  velocities  of  different 
ions  vary  from  about  three-quarters  of  a  centimetre 
per  second  in  the  case  of  carbon  dioxide,  to  about 
seven  centimetres  per  second  in  the  case  of  hydro- 
gen. The  velocity  of  the  negative  ion  is,  in 
general,  appreciably  greater  than  that  of  the  posi- 
tive ion,  the  ratio,  unity  for  carbon  dioxide,  rising 
to  1.24  for  air  and  oxygen. 

We  should  expect  the  velocity  of  an  ion  to  be 
inversely  proportional  to  the  pressure  of  the  gas, 
and  this  has  been  found  to  be  the  case  with  the 
positive  ions.  The  mobility  of  the  negative  ions, 
on  the  other  hand,  increases  with  decreasing  pres- 
sure much  faster  than  this  expectation  justifies, 
and  at  low  pressures,  100  millimetres  of  mercury 


CONDUCTION  THROUGH  GASES       155 

and  less,  the  change  is  very  marked.  This  result 
indicates  an  alteration  in  the  nature  of  the  ions 
themselves,  and  justifies  the  belief  that  they  must 
possess  more  complex  structures  at  high  than  at 
low  pressures. 

We  shall  see  later  that,  at  the  very  low  pressures 
which  exist  in  good  vacuum  tubes,  it  is  possible  to 
estimate  the  absolute  mass  of  the  ions,  with  the 
remarkable  result  that,  whereas  the  mass  of  the 
positive  ion  appears  to  be  much  the  same  as  the 
mass  of  an  atom,  the  mass  of  the  negative  ion 
comes  out  about  the^mousandth  part  of  the  mass 
of  the  lightest  atom  known  to  chemistry,  that  of 
hydrogen.  The  decrease  of  the  ionic  velocity  at 
low  pressures  probably  indicates  an  approach  to 
this  state  of  low  ionic  mass. 

A  similar  decrease  in  the  size  of  the  negative  ion, 
compared  with  that  of  the  positive,  is  produced  by 
raising  the  temperature.  H.  A.  Wilson  found 
that,  at  2000°  C.,  the  velocity  of  the  negative 
ions,  produced  by  salts  volatilised  in  flames, 
was  seventeen  times  greater  than  the  velocity  of 
the  positive  ions. 

The  problem  of  determining  the  dimensions  of 
the  ions  at  atmospheric  pressure  has  been  attacked 
by  measuring  their  rates  of  diffusion  into  non- 
ionized  gases.  The  rate  of  diffusion  of  a  gas 
depends  on  the  mass  of  its  molecule,  and  experi- 


156  PHYSICAL  SCIENCE 

ments  show  that  the  mass  of  an  ion  at  atmospheric 
pressure  is  considerably  greater  than  that  of  the 
molecule  of  an  ordinary  gas. 

All  these  results  may  be  explained  by  the  theory 
that  the  normal  process  of  gaseous  ionization 
consists  in  the  detachment  from  an  atom  of  the 
gas  of  a  minute  particle,  called  by  Professor  J.  ]. 
Thomson  a  corpuscle.  At  extremely  low  pres- 
sures the  corpuscle  constitutes  the  negative  ion, 
and  the  atom  or  molecule  from  which  it  has  been 
separated  forms  the  positive  ion.  As  the  pressure 
rises,  neutral  molecules  become  attached  to  the 
ions,  probably  by  virtue  of  the  electric  forces,  and 
collect  round  the  original  ion,  which  constitutes 
the  nucleus.  These  complex  systems  form  the 
ions  of  gases  at  atmospheric  pressures. 

The  presence  of  gaseous  ions  may  be  inferred 
from  the  phenomena  of  current  conduction  through 
the  gases,  but  the  existence  of  charged  particles 
of  greater  than  molecular  dimensions  has  been 
demonstrated  by  Mr.  C.  T.  R.  Wilson  in  a  very 
direct  and  striking  manner.  Long  ago  Aitken 
showed  that  the  condensation  of  drops  of  water 
from  air  saturated  with  aqueous  vapour  was  much 
helped  by  the  presence  of  particles  of  dust  ;  in 
the  absence  of  dust,  considerable  supersaturation 
could  be  attained  before  condensation  set  in. 


FIG.  28. — CONDENSATION  OF  CLOUD  ON  GASEOUS  IONS 
(Mr.  C.  T.  R.   Wilson} 

To  face  page  157 


CONDUCTION  THROUGH  GASES       157 

Each  particle  of  dust  forms  a  nucleus,  round 
which  collect  molecules  of  water  ;  and,  when  the 
drops  have  grown  to  a  sufficient  size,  they  fall, 
carrying  down  the  dust  particle  also.  In  this  way 
finally  the  air  is  freed  from  the  presence  of  dust, 
and  to  this  action,  on  a  large  scale,  we  must 
attribute  partially  the  clearness  of  the  atmosphere 
after  a  downfall  of  rain. 

Wilson  devised  an  apparatus  whereby  air  could 
be  subjected  to  a  sudden  expansion.  By  this  means 
it  was  cooled  ;  and,  if  previously  saturated  with 
water  vapour,  any  desired  degree  of  supersaturation 
could  be  obtained  by  adjusting  the  amount  of  ex- 
pansion. By  repeated  expansions,  the  dust  particles 
were  removed,  and  any  further  expansions  then 
produced  only  a  few  drops  of  water.  If,  however, 
when  the  air  had  thus  been  depleted  of  possible 
nuclei,  Rontgen  rays  or  other  ionizing  agency  were 
allowed  to  act  on  the  gas,  instead  of  these  few 
drops,  a  dense  cloud  was  once  more  obtained  by 
the  same  expansion.  This  cloud  was  not  formed 
if  the  ions  were  removed  previously  by  an  electric 
field,  or  by  some  other  means. 

Fig.  28  is  a  photograph  of  one  of  Mr.  Wilson's 
clouds,  illuminated  by  a  beam  of  light  from  an 
electric  lantern.  The  nuclei  in  this  case  were  the 
ions  produced  by  a  piece  of  radium  contained  in 
the  tube  seen  to  the  right  of  the  glass  cloud- 


158  PHYSICAL  SCIENCE 

chamber.  The  cloud  has  settled  down  to  the 
lower  part  of  the  hemispherical  chamber,  and  its 
sharply-defined  upper  surface  is  clearly  visible. 
The  expansion  is  effected  by  the  movement  of  a 
piston  within  the  vertical  brass  cylinder,  the  lower 
part  of  which  is  put  suddenly  into  communica- 
tion with  the  exhausted  vessel  seen  lying  on  the 
table. 

In  1893,  Professor  Thomson  had  shown  that, 
in  causing  condensation,  negative  electrification 
was  more  effective  than  positive,  and  Wilson,  in 
1899,  further  examined  this  point.  He  found  that, 
while  negative  ions  produced  condensation  of  a 
cloud  when  the  volume  of  the  gas  was  increased 
in  the  ratio  of  i  :  1.28,  positive  ions  did  not  cause 
an  equal  effect  till  the  expansion  reached  1.31. 
It  is  possible  that  this  difference  may  have  an 
important  meteorological  significance.  If,  as  there 
is  reason  to  suppose,  the  atmosphere  sometimes 
contains  a  considerable  number  of  gaseous  ions, 
an  expansion  or  fall  of  temperature  would  result 
in  the  formation  of  drops  of  water  round  the 
negative  ions  sooner  than  round  the  positive  ions. 
The  negative  ions  thus  would  be  removed  first, 
and  the  air  would  be  left  with  an  excess  of  posi- 
tive electrification.  It  is  not  unlikely  that  the 
origin  of  the  commonly  observed  potential  of  the 
atmosphere,  positive  relatively  to  that  of  the  earth, 


CONDUCTION  THROUGH  GASES       159 

is,  partially  at  any  rate,  to  be  found  in  this  selec- 
tive withdrawal  of  the  negative  ions. 

If  the  ionization  be  not  too  intense,  it  is  possible 
to  remove  completely  the  ions  from  air  by  means 
of  a  single  expansion.  Each  ion  will  then  be 
the  nucleus  of  a  water-drop  ;  and,  since  the  amount 
of  water  left  in  the  air  must  be  just  that  required 
for  the  equilibrium  of  saturation,  the  quantity  of 
water  removed  by  the  falling  cloud  can  be 
calculated.  This  amount  of  water  is  constant  for 
a  given  expansion,  and  the  number  of  ions  present 
must  therefore  be  the  factor  which  determines  the 
size  of  the  drops.  Minute  drops,  the  constituent 
parts  of  the  artificial  cloud  or  fog  under  considera- 
tion, fall  very  slowly,  and  Sir  George  Stokes  has 
shown  how  their  size  may  be  calculated  from  the 
rate  of  their  fall.  The  cloud  settles  down  at  a 
steady,  well-marked  pace,  which  can  readily  be 
observed  by  watching  the  upper  surface  as  seen 
in  Fig.  28.  This  measurement  gives  the  average 
size  of  each  drop  ;  and,  since  the  total  mass  of  all 
the  drops  can  be  calculated  from  the  expansion, 
the  total  number  of  drops,  and  therefore  of  ions, 
can  be  deduced  approximately. 

J.  J.  Thomson  has  used  this  method  to  determine 
the  electric  charge  on  a  gaseous  ion.  The  current 
through  the  gas  is  given  by  the  product  of  the 
number  of  ions,  the  charge  carried  by  each,  and 


i6o  PHYSICAL  SCIENCE 

the  velocity  with  which  they  move.  The  velocity, 
as  we  have  said,  can  be  determined  for  a  known 
electro-motive  force  ;  and,  by  measuring  the  resul- 
tant current  with  an  electrometer,  and  rinding  the 
number  of  ions  by  Wilson's  method,  the  ionic 
charge  was  estimated.  Within  the  limits  of  ex- 
perimental error  it  was  found  to  be  the  same  as 
the  charge  on  an  ion  in  liquid  electrolysis,  and  to 
have  the  value  3.4Xio~10  electro-static  units. 
The  importance  of  this  result  will  appear  later. 

We  must  now  consider  another  series  of  experi- 
ments in    which  these    particles,  called    variously 
ions,    corpuscles,    and    ultra-atomic     bodies,   have 
been  detected.     The  investigations  were  original!] 
planned  and  carried  out  to  determine  other  points 
of  interest,  and  only  comparatively  recently  hav< 
they  been  used   to   elucidate  the   present   subjecl 
matter. 

An  electric  machine  capable  of  yielding  sparks 
was  invented  many  years  ago  during  the  eighteentl 
century  ;  and  the  question  soon  arose  whether  sucl 
sparks  were  of  the  same  nature  as  the  lightning 
flash — whether  the  roll  of  the  thunder  was  bul 
the  reiterated  crackle  of  the  stupendous  electri< 
machine  of  the  atmosphere  working  amid  th< 
convolutions  of  the  clouds.  The  question  was 
answered  by  Franklin,  who  floated  a  kite  in  th< 


CONDUCTION  THROUGH  GASES      161 

air,  and,  when  the  string  was  made  a  conductor 
by  a  shower  of  rain,  was  able  to  draw  the  con- 
firming sparks  from  its  lower  end. 

A  very  great  electric  force  is  required  to  maintain 
a  visible  discharge  through  a  few  centimetres  of 
air  at  the  atmospheric  pressure,  and  the  initial 
force  needed  to  start  the  process  is  still  larger.  It 
was  soon  found,  however,  that  a  reduction  of 
pressure  facilitated  the  passage  of  the  spark,  and 
that  it  was  much  easier  to  send  the  discharge 
through  a  vessel  from  which  the  air  had  been 
partially  exhausted  by  means  of  an  air-pump.  To 
illustrate  this,  platinum  wires,  to  act  as  electrodes, 
are  sealed  into  little  glass  tubes  containing  air  at 
low  pressure.  For  many  years  these  vacuum 
tubes,  as  they  are  called,  were  the  electrical  play- 
things of  the  laboratory  and  popular  lecture-room. 
Recent  discoveries  have  raised  them  from  the 
position  of  scientific  toys  to  the  rank  of  pieces  of 
apparatus  whereby  have  been  made  some  of  the 
greatest  discoveries  in  physical  knowledge  that 
the  present  generation  has  seen. 

Through  such  a  tube,  in  which  the  pressure  of 
the  air  is  only  a  small  part  of  an  atmosphere,  a 
discharge  may  readily  be  passed  by  the  aid  of  a 
voltaic  battery  and  an  induction  coil,  or  by  the  use 
of  an  influence  electric  machine.  As  in  liquid 
conductors,  the  electrode  by  which  the  current 

L 


162  PHYSICAL  SCIENCE 

enters  is  called  the  anode,  and  that  by  which  it 
leaves,  the  cathode.  Starting  from  the  cathode, 
we  first  see  a  bright  glow  covering  its  surface,  then 
a  dark  space,  succeeded  by  a  second  dark  space, 
beyond  which  is  a  luminous  column  reaching  to 
the  anode.  Within  certain  limits  of  pressure  and 
strength  of  current,  this  positive  column,  as  it  has 
been  called,  shows  fluctuating  striations.  If  the 
length  of  the  tube  be  increased,  it  is  this  positive 
column  alone  which  increases  with  it ;  the  two 
dark  spaces,  and  the  negative  glow,  vary  very 
little  with  the  length  of  the  tube. 

The  effect  of  very  high  vacua  on  the  electric 
discharge  was  first  systematically  investigated  by 
Sir  William  Crookes.  As  the  air  is  gradually 
removed,  it  is  found  that  the  dark  space  nearesl 
the  cathode,  now  known  as  the  Crookes'  dark 
space,  gradually  extends,  until  eventually  it  fills 
the  whole  tube.  At  this  stage,  green  phosphor- 
escent effects  begin  to  appear  on  the  anode  and 
on  the  glass  opposite  the  cathode.  If  a  solid 
object,  such  as  a  screen  of  mica,  be  interposed 
between  the  glass  and  the  cathode,  a  sharp  shadow 
is  seen,  showing  from  its  position  that  rays  capable 
of  producing  phosphorescence  proceed  in  straight 
lines  from  the  cathode.  These  cathode  rays 
possess  energy,  for  a  light  windmill  placed  in 
their  path  can  be  made  to  rotate ;  moreover,  they 


CONDUCTION  THROUGH  GASES       163 

are  deflected  by  a  magnet,  in  the  same  direction 
as  would  be  negatively  electrified  particles,  travel- 
ling in  the  course  of  the  rays.  For  this  reason, 
Crookes  and  other  English  observers  from  the 
first  contended  that  the  cathode  rays  were  to  be 
regarded  as  a  flight  of  negatively  electrified  mate- 
rial particles ;  while,  on  the  contrary,  it  was  believed 
for  some  time  in  Germany,  where  many  experi- 
ments were  also  made,  that  the  cathode  rays,  like 
those  of  ordinary  light,  were  of  the  nature  of 
aethereal  waves. 

In  the  year  1895,  Professor  Rontgen  of  Munich 
made  the  first  of  the  sensational  discoveries  in 
physical  science  for  which  the  last  few  years  have 
been  remarkable.  Many  other  recent  investiga- 
tions have  been  as  interesting,  and  several  have 
more  profoundly  modified  our  outlook  on  Nature, 
but  few  have  struck  so  readily  the  imagination  of 
the  plain  man  as  the  revelation  of  the  skeleton 
within  the  living  flesh. 

The  origin  of  this  discovery  may  be  said  to 
have  been  almost  accidental.  Rontgen  noticed 
that  photographic  plates,  kept  under  cover  in  the 
neighbourhood  of  a  highly  exhausted  tube  through 
which  electric  discharges  were  passing,  became 
fogged,  as  though  they  had  been  exposed  to  light. 
He  investigated  this  effect,  and  found  that,  when 
cathode  rays  impinged  either  on  the  glass  of  the 


164  PHYSICAL  SCIENCE 

tube;  or  on  the  anode,  or  on  any  metallic  plate 
within  the  tube,  a  type  of  radiation  was  produced 
which  would  penetrate  many  substances  opaque 
to  ordinary  light.  Dense  bodies,  like  metal  or 
bone,  absorbed  the  rays  more  fully  than  did 
lighter  materials,  such  as  leather  or  flesh,  and 
Rontgen,  at  once  putting  this  discovery  to  some 
purpose,  was  able  to  photograph  the  coins  in  his 
purse  and  the  bones  in  his  hand. 

Given  the  rays,  the  mechanical  contrivances 
required  to  demonstrate  their  effects  are  not 
elaborate.  Rontgen  rays  produce  phosphor- 
escence on  screens  of  barium  platino-cyanide 
and  other  similar  salts,  and,  by  using  these  screens 
in  place  of  a  photographic  plate,  objects,  usually 
hidden  from  our  eyes,  may  be  made  visible. 

A  remarkable  property  of  the  rays  is  their  power 
of  converting  the  air  and  other  gases  through  which 
they  pass  into  conductors  of  electricity.  In  ordi- 
nary circumstances,  as  was  pointed  out  in  the 
earlier  part  of  this  chapter,  air  is  an  almost  perfect 
insulator  ;  and  an  electrified  body  exposed  to  it, 
while  shielded  from  other  sources  of  leakage,  loses 
its  charge  with  extreme  slowness.  If,  however, 
Rontgen  rays  are  passing  through  the  air  in  the 
neighbourhood  of  the  electrified  body,  the  charge 
quickly  disappears. 

For    several    years    after    their    discovery,    the 


CONDUCTION  THROUGH  GASES      165 

physical  nature  of  the  Rontgen  rays  was  widely 
discussed,  and,  for  a  long  time,  no  general  con- 
sensus of  opinion  was  reached.  Their  photo- 
graphic effects  and  the  fluorescence  they  produced 
on  suitable  screens  suggested  that,  like  ordinary 
light,  they  were  to  be  regarded  as  waves  in  the 
luminiferous  aether.  The  power  they  possess  of 
penetrating  some  opaque  substances  does  not 
forbid  such  an  assumption  ;  for  a  difference  in  the 
wave-length,  or  in  the  period  of  vibration,  is  suffi- 
cient to  produce  marked  differences  in  the  pene- 
tration of  ordinary  light.  Glass,  transparent  to 
the  visible  rays,  is  opaque  to  those  invisible  rays 
of  longer  wave-length,  which  possess  great  heating 
power — hence  its  use  in  fire-screens  ;  while  a  solu- 
tion of  iodine  in  bisulphide  of  carbon  is  opaque  to 
luminous  radiation,  but  allows  the  long  waves  to  pass. 
Rontgen  rays  are  not  refracted  like  ordinary 
light,  and  very  little  trace  of  regular  reflection  has 
been  detected.  Moreover,  it  is  only  just  recently, 
with  great  difficulty,  that  they  have  been  found 
to  show  signs  of  such  a  typical  property  as 
polarisation.  Two  plates  of  tourmaline  seem  to 
be  as  transparent  to  the  rays  when  the  axes  of 
the  crystals  are  crossed  as  when  the  axes  are 
parallel.  Such  indications  as  these  seem  to  be 
inconsistent  with  the  iderVtity  in  nature  between 
Rontgen  rays  and  ordinary  light. 


166  PHYSICAL  SCIENCE 

On  the  other  hand,  the  rays  suffer  no  deviation 
when  acted  on  by  a  magnetic  or  by  an  electric 
field  of  force,  a  result  which  indicates  that  they 
are  not  projected  particles  carrying  electric  charges. 
In  this  particular,  they  must  carefully  be  distin- 
guished from  their  creative  agency — from  the  flight 
of  negative  particles  or  cathode  rays  which,  by 
impact  on  glass  or  metal,  give  rise  to  this  new 
type  of  radiation. 

In  the  year  1896,  Sir  George  Stokes  suggeste( 
that  an  explanation  should  be  sought  in  the  hypo- 
thesis that  Rontgen  rays  were  single  pulses  travel- 
ling through  the  aether.  Ordinary  light  is  to  b< 
represented  as  a  series  of  regular  waves,  succeed- 
ing each  other  at  periodic  intervals,  many  thousan< 
waves,  almost  exactly  similar  to  each  other,  follow- 
ing in  order  in  a  minute  fraction  of  a  secon< 
According  to  this  view,  Rontgen  rays  must  b< 
regarded  as  single  disturbances,  propagated  wit! 
the  same  velocity  as  light,  but  not  followed  by 
train  of  waves.  The  thickness  of  the  pulse,  ii 
which  the  whole  disturbance  is  concentrated, 
considerably  smaller  than  the  wave-length  of  an; 
visible  light. 

On  Maxwell's  theory,  now  universally  accepte< 
light  is  explained  as  a  series  of  electro-magneti< 
waves;  and  we  must  therefore  imagine  the  Ront- 
gen pulses  to  be  electro-magnetic  also.     But, 


CONDUCTION  THROUGH  GASES      167 

we  have  said,  Rontgen  rays  are  produced  when 
a  cathode  ray  strikes  a  solid  object ;  and,  if  we 
take  the  cathode  rays  to  be  streams  of  electrified 
particles,  it  may  be  shown  that  electro-magnetic 
pulses  will  be  started  by  their  impact. 

Let  us  examine  the  electric  properties  of 
these  moving  particles  by  means  of  the  concep- 
tion of  tubes  of  force,  a  conception  which  we 
owe  to  the  instinctive  insight  of  Faraday.  A 
small  electrified  body,  carrying,  let  us  suppose, 
a  negative  charge,  is  well  known  to  attract 
other  bodies  in  the  neighbourhood  when  those 
bodies  are  positively  electrified,  and  to  repel 
them  if  their  charges  be  negative.  The  forces 
which  the  first — the  negatively  electrified — body 
exerts  may  be  represented  by  a  series  of  lines, 
drawn  everywhere  so  as  to  lie  in  the  direction 
of  the  force  on  a  positively  electrified  par- 
ticle. 

The  distribution  of  these  electric  lines  of  force 
can  be  investigated  theoretically,  the  laws  of  force 
being  known,  but  it  is  not  easy  to  illustrate  them 
experimentally.  On  the  other  hand,  the  corre- 
sponding magnetic  lines  can  be  rendered  visible 
and  mapped  out  by  a  familiar  experiment,  which, 
indeed,  first  suggested  to  Faraday  his  conception 
of  lines  or  tubes  of  force.  If  the  poles  of  a 
horse-shoe  magnet  be  placed  beneath  a  sheet  of 


i68 


PHYSICAL  SCIENCE 


cardboard,  over  which  iron  filings  are  sprinkled, 
a  picture  of  the  magnetic  lines  of  force  is  formed 
by  the  filings  (Fig.  29).  Under  the  influence  of 
the  magnetic  field,  each  filing  becomes  a  little 
magnet,  and  attracts  others,  forming  chains  of 
filings  which  lie  everywhere  in  the  direction  of 


FIG.  29. 

the  magnetic  force.  Where  the  force  is  strong, 
the  filings  cluster  thickly  ;  where  the  force  is 
weak  few  filings  are  to  be  seen.  Thus  a  com- 
plete representation  of  the  lines  of  magnetic  force 
is  obtained. 

The  laws   of   force   are   of  the  same  form  for 
electric     charges     as     for    magnetic     poles,     and 


CONDUCTION  THROUGH  GASES      169 

the  filings  in  Fig.  30  represent  also  the  direction 
and  distribution  of  the  electric  lines  or  tubes  of 
force  in  the  neighbourhood  of  two  electric 
charges  of  opposite  signs.  Here  we  have  two 
charges  ;  but,  for  an  isolated  charged  body,  the 
lines  of  electric  force  must  evidently  be  radial, 
as  shown  in  the  region  near  the  particle  O  in 
Fig.  30,  when  op 

represents  one  such  P 

line  of  force  pro- 
ceeding from  the 
electric  charge  at 
O.  If  the  electrified 
particle  be  travel- 
ling forwards,  in 
the  direction  of  the 
arrow,  it  carries  its 
lines  of  force  with 
it  ;  and,  unless  the 
particle  be  moving 
with  a  velocity  very  nearly  equal  to  the  velo- 
city of  light,  the  distribution  of  the  lines  is 
unaltered  ;  they  still  are  uniformly  placed  radii, 
proceeding  from  the  particle  as  centre.  It  was 
predicted  by  Maxwell,  and  has  been  experiment- 
ally demonstrated  by  Rowland,  Fender,  and 
others,  that  a  moving  charged  body  behaves 
as  a  current  of  electricity.  A  current  produces 


o     o' 


FIG.   30, 


1 70  PHYSICAL  SCIENCE 

a  magnetic  force,  and  thus  a  magnetic  field  is 
produced  by  the  moving  particle  o  of  Fig.  30. 
In  this  way  we  see  that  whenever  electric  tubes 
of  force  are  moving,  there  exists  a  magnetic 
force  at  right  angles,  both  to  their  length  and 
to  their  direction  of  motion. 

Now  let  us  imagine  the  moving  particle  to  be 
stopped  suddenly.  If  a  change  could  be  pro- 
pagated instantaneously  throughout  all  space,  the 
lines  of  force  would  at  once  stop  also.  But  a 
change  in  electro-magnetic  properties  can  only 
be  propagated  with  the  speed  of  an  electro- 
magnetic wave,  that  is,  with  the  velocity  of 
light.  Thus,  when  a  moving  electrified  particle 
is  arrested  suddenly,  a  pulse  of  electro-magnetic 
force  starts  from  the  particle  as  its  centre,  and 
spreads  out  in  circles,  rectifying  the  distribu- 
tion of  the  lines  of  force  as  it  goes.  The 
effect  is  shown  in  Fig.  30.  If  the  particle  had 
not  been  stopped,  at  the  end  of  an  interval  of 
time,  t,  it  would  have  reached  some  new  posi- 
tion o',  and  the  lines  of  force  would  be  radii 
from  this  point  as  centre.  Beyond  the  sphere 
reached  by  the  rectifying  pulse,  the  lines  of  force 
will  still  be  moving  parallel  to  O^,  and,  at  the 
instant  considered,  will  be  radii  of  the  point  o', 
while  behind  the  spherical  pulse  the  lines  will 
be  at  rest,  and  will  be  radii  of  the  point  at 


CONDUCTION  THROUGH  GASES      171 

which  the  particle  is  stopped.  The  lines  of  force 
must  be  continuous  ;  and  therefore,  in  the  pulse 
itself,  the  lines  must  run  in  some  direction  such 
as  Pq  in  the  figure.  The  electric  force  near 
Pq  has  then  a  component  at  right  angles  to  the 
direction  of  propagation  of  the  disturbance,  that 
is,  at  right  angles  to  the  radial  lines.  Whenever 
a  Faraday  tube  of  electric  force  moves,  it  pro- 
duces a  magnetic  force  at  right  angles  both  to 
its  length  and  to  its  direction  of  motion,  and 
thus  the  line  of  force  Pq  within  the  pulse  pro- 
duces a  magnetic  force  at  right  angles  to  the 
plane  of  the  paper.  Now  the  waves  of  light, 
and,  to  pass  to  much  greater  wave-lengths,  the 
waves  used  in  wireless  telegraphy,  are  aethereal 
waves  of  electro-magnetic  force  so  arranged  that 
the  electric  and  magnetic  forces  are  at  right 
angles  to  each  other,  and  both  at  right  angles 
to  the  direction  of  propagation  of  the  waves.  It 
follows  that  the  pulse,  indicated  by  our  figure 
as  spreading  out,  consequent  on  its  arrest,  from  a 
moving  electrified  particle,  is  a  pulse  of  the  same 
nature  as  the  waves  of  light,  with  this  excep- 
tion, that,  instead  of  a  series  of  regular  periodic 
waves,  it  consists  of  a  single  expanding  shell  of 
electro-magnetic  force.  Compared  even  with  the 
minute  wave-length  of  ordinary  light,  the  thick- 
ness of  the  shell  is  exceedingly  small,  and  depends 


172  PHYSICAL  SCIENCE 

on  the  character  of  the  arrest ;  it  becomes  smaller 
the  more  sudden  the  stoppage  of  the  particle. 
Electro -magnetic  pulses  then  will  arise  in  the 
circumstances  which  are  known  to  exist  when 
Rontgen  rays  appear,  and  Stokes  has  shown 
theoretically  that  such  pulses  will  possess  many 
of  the  properties  characteristic  of  Rontgen  rays. 

The  successful  explanation  of  the  production 
and  properties  of  Rontgen  rays  is  strong  evidence 
in  favour  of  that  view  of  cathode  rays  which 
regards  them  as  negatively  electrified  particles, 
shot  out  with  great  velocity  from  the  neighbour- 
hood of  the  cathode.  But  much  other  evidence 
tending  in  the  same  direction  has  come  to  light, 
and  nowadays  no  one  doubts  the  material  nature 
of  cathode  rays. 

Direct  evidence  of  the  negative  charge  carried 
by  the  cathode  rays  was  given  by  experiments 
of  Perrin.  He  showed  that  when  the  rays  were 
deflected  by  a  magnet  so  that  they  fell  on  an 
insulated  metal  cylinder  connected  with  an  elec- 
trometer, a  strong  negative  electrification  was 
imparted  to  the  system.  When  the  rays  fell  on 
other  parts  of  the  bulb,  this  electrification  was 
not  observed. 

A  less  direct  but  more  interesting  method  is 
due  to  J.  J.  Thomson.  In  the  glass  apparatus 


CONDUCTION  THROUGH  GASES       173 

shown  in  Fig.  3 1 ,  which  is  a  photograph  of  the 
tube  actually  used  in  the  experiment,  the  left- 
hand  terminal  of  the  induction  coil  is  connected 
with  the  cathode,  the  right-hand  terminal  with  a 
thick  metallic  disc  which  acts  as  the  anode. 
Through  the  anode,  and  through  a  second  thick 
disc  connected  with  the  earth  by  the  wire  going 
to  the  bottom  of  the  photograph,  are  bored  in 
sequence  two  holes  about  a  millimetre  in  diameter. 
A  thin  pencil  of  cathode  rays  is  thus  obtained 
beyond  the  second  disc.  These  rays  then  pass 
between  the  two  metallic  plates,  seen  in  the 
wider  part  of  the  tube,  which  can  be  connected 
with  the  poles  of  a  voltaic  battery  by  means  of 
the  wires  passing  to  the  right.  An  electric  force 
of  known  amount  can  thus  be  applied  to  the 
cathode  rays.  When  that  force  is  sufficient,  the 
path  of  the  rays  is  deflected,  and  the  magnitude 
of  this  effect  can  be  determined  by  observing 
the  deflection  of  the  spot  of  fluorescent  light  at 
the  right-hand  end  of  the  apparatus.  It  is  well 
known  that  the  cathode  rays  are  also  deflected 
by  a  magnetic  field,  and  this  effect  too  can  be 
measured  in  the  same  apparatus.  Both  these 
deflections  are  to  be  expected  if  the  rays  consist 
of  moving  electrified  particles;  and  the  directions 
of  the  deflections  are  such  that  the  electrification 
must  be  that  to  which  is  conventionally  given 


174  PHYSICAL  SCIENCE 

the  negative  sign.  No  system  of  aethereal  waves 
yet  described  would  give  these  results. 

The  conclusions  drawn  from  the  results  of  these 
experiments  are  of  extreme  importance.  In  analys- 
ing the  deflections  of  the  particles  three  things  are 
involved:  (i)  the  velocity;  (2)  the  mass  ;  and  (3) 
the  electric  charge.  For  both  deflections,  the 
electric  and  magnetic,  the  two  last  quantities  appear 
as  the  ratio  ejm — that  is,  the  charge  divided  by  the 
mass.  If  we  treat  this  ratio  as  a  single  quantity, 
we  find  ourselves  with  two  unknown  values  to  be 
determined  by  the  two  experiments,  the  one  on 
the  magnetic,  and  the  other  on  the  electric 
disturbances.  Both  the  unknown  quantities — to 
wit,  the  velocity  and  the  ratio  ejin — can  therefore 
be  found  from  the  results  of  the  experiments. 

When  a  magnetic  force  is  applied,  the  spot  of 
phosphorescent  light  in  the  tube  of  Fig.  31  is 
drawn  out  into  a  band  of  appreciable  length. 
This  result  is  a  consequence  of  a  difference  in 
velocity  of  the  rays  :  in  any  one  discharge,  rays 
are  found  with  a  considerable  range  of  velocity, 
and  therefore  are  deflected,  according  to  their 
velocities,  through  a  series  of  different  angles. 

The  following  table  gives  some  of  the  results  of 
Professor  Thomson's  experiments,  and  shows  the 
mean  values  of  the  velocity,  v,  in  centimetres  per 
second,  and  of  the  ratio  mje  for  cathode  rays,  m 


CONDUCTION  THROUGH  GASES       175 

being    expressed   in   grammes,    and    e  in   electro- 
magnetic units  of  electricity. 

Gas.                                     v.  mje. 

Air           ....  2.8x10°  I.2XIO-7 

Hydrogen       .         .         .  2.5  xio9  i.5Xio~7 

Carbonic  acid          .         .  2.2x10°  i.5xio~7 

Thus,  within  the  limits  of  experimental  error, 
the  results  are  independent  of  the  nature  of  the 
residual  gas  left  in  the  vacuum  tube.  Moreover, 
in  these  experiments,  and  in  a  further  series  due 
to  H.  A.  Wilson,  the  results  were  shown  to  be 
the  same  whatever  metal  was  used  to  form  the 
cathode.  In  all  circumstances  the  mean  velocity 
is  very  high,  being  about  one-twelfth  that  of  light, 
and  the  mean  value  of  mje  is  1.3  x  io~~7,  which 
makes  the  reciprocal  ratio  e/m  about  7.7  x  io6. 

Now  in  liquid  electrolytes,  the  passage  of  one 
electro-magnetic  unit  of  electricity  evolves  io~4 
gramme  of  hydrogen.  Thus,  in  this  case,  the 
ratio  e/m  is  about  io4,  or  from  seven  to  eight 
hundred  times  less  than  its  value  for  the  negative 
particle  in  a  cathode  ray. 

But,  as  we  have  already  seen  (p.  157),  by  an 
application  of  C.  T.  R.  Wilson's  beautiful  experi- 
ments on  the  electric  formation  of  clouds,  Thom- 
son has  proved  that  the  individual  charge  on  all  the 
gaseous  ions  examined  is  the  same  as  the  charge 
on  the  ions  in  liquid  electrolysis.  Although  the 


i;6  PHYSICAL  SCIENCE 

cathode  ray  particles  themselves  could  not  be 
investigated  in  this  way,  there  seems  no  reason  to 
suppose  that  they  are  exceptions  to  an  otherwise 
universal  rule.  If  then,  e  is  the  same  both  for 
gases  and  for  liquids,  m  must  be  different  ;  it  must 
be  from  seven  hundred  to  a  thousand  times  less 
for  the  cathode  ray  particle  than  for  the  hydrogen 
atom. 

There  is  another  way  of  performing  the  same 
calculation  which  may  be  of  interest  in  this 
connection.  The  charge  on  a  ion  is  (p.  160)  about 
3.4  X  io~10  electro-static  units,  a  number  which 
must  be  divided  by  the  velocity  of  light,  3  x  io10 
centimetres  per  second,  if  we  wish  to  convert 
it  into  electro-magnetic  units.  The  result  is 
i.i  x  io~20,  as  the  value  of  e.  But  ejm  is  about 
7.7  x  io6,  and  thus  m,  the  mass  of  the  corpuscle, 
is  about  T  x  i  o~26  gramme. 

The  kinetic  theory  of  gases  enables  us  to 
estimate  the  number  of  molecules  in  a  cubic 
centimetre  of  gas  as  about  2  x  io19  or  io20.  The 
number  of  atoms  of  hydrogen  will  be  double  the 
number  of  molecules  ;  and,  as  a  mean  value,  we 
may  take  io20  to  be  about  the  number  of  atoms 
in  a  cubic  centimetre.  This  volume  of  hydrogen 
weighs  9  x  io~5  gramme,  which  makes  the  mass 
of  a  single  atom  about  9  x  io~23  gramme.  Com- 
paring this  with  ^  x  io~26,  the  mass  of  the 


CONDUCTION  THROUGH  GASES      177 

corpuscle,  we  again  find  that  the  corpuscle  has 
a  mass  of  about  the  eight-hundredth  part  of  that 
of  the  atom  of  hydrogen. 

Similar  values  have  been  obtained  for  the  mass 
of  the  negative  particles  when  produced  in  other 
ways.  In  one  case,  that  of  the  ions  due  to  the 
incidence  at  a  low  pressure  of  ultra-violet  light  on 
metals,  both  e  and  elm  have  been  measured  for  the 
same  particles.  A  zinc  plate  is  illuminated  with 
ultra-violet  light,  and  placed  opposite  to  and  parallel 
with  a  second  metallic  plate  connected  with  an 
electrometer,  the  gas  surrounding  the  apparatus 
being  exhausted  to  a  very  low  pressure.  An 
electric  force  is  established  between  the  two 
plates,  and  the  negative  ions,  produced  at  the 
zinc  plate,  are  by  this  force  urged  towards  the 
second  plate.  If  no  other  agency  were  at  work, 
all  the  negative  ions  would  reach  the  second  plate, 
and  transfer  their  charges  to  the  electrometer. 
Now  let  us  imagine  that  a  magnetic  force  is 
applied  at  right  angles  to  the  electric  force  and 
parallel  to  the  planes  of  the  plates.  The  magnetic 
force  will  deflect  the  negative  particles  from  their 
original  straight  course,  and  their  path  becomes 
a  cycloid.  They  travel  out  from  the  zinc  plate, 
curve  round,  and  approach  it  again.  If  the 
second  plate  is  placed  near  enough  to  the  first 
to  intercept  this  curved  orbit,  all  the  ions  will 

M 


1 78  PHYSICAL  SCIENCE 

still  reach  the  plate  connected  with  the  electro- 
meter, and  the  rate  at  which  it  gains  negative 
electricity  will  not  be  affected  by  the  presence  of 
the  magnetic  field.  If,  however,  the  electrometer 
plate  be  moved  away  from  the  zinc  plate  till  it 
lies  beyond  the  path  of  the  ions,  it  will  receive 
none  of  them,  and  the  establishment  of  the 
magnetic  force  should  completely  stop  the  supply 
of  negative  electricity  to  the  electrometer.  If  X 
be  the  electric  force  and  H  the  magnetic  force, 
theory  shows  that  no  ions  should  cross  the  space 
between  the  plates  if  the  distance  between  them 
exceeds  2Xm/eH2,  while  below  that  distance  the 
addition  of  the  magnetic  force  H  should  produce 
no  effect  on  the  rate  of  gain  of  negative  charge 
by  the  electrometer. 

The  experiments  which  Thomson  carried  out 
by  this  method  showed  that  no  such  sudden 
change  could  be  produced.  As  the  distance  was 
diminished,  or  the  magnetic  field  increased,  at 
first  the  effect  of  putting  on  or  taking  off  the 
magnetic  force  was  small.  Then  a  stage  is 
reached  at  which  a  considerable  effect  is  pro- 
duced; while  finally,  in  a  third  stage,  the  mag- 
netic force  uts  off  almost  all  the  ions  from 
the  electrometer  plate.  This  somewhat  gradual 
change  is  explained  if  we  suppose  that  the  nega- 
tive ions  are  not  all  formed  at  the  surface  of  the 


CONDUCTION  THROUGH  GASES       179 

zinc  plate,  but  that,  as  the  primary  ions  there 
produced  move  forward  under  the  action  of  the 
electric  force,  they  produce  new  ions  by  their 
collisions  with  the  molecules  of  the  gas.  The  ions 
are  thus  formed,  not  exclusively  at  the  surface  of 
the  plate,  but  throughout  a  thin  layer  of  gas  near 
the  plate.  This  secondary  production  of  ions  by 
primary  ions  moving  with  high  velocities  occurs 
in  many  other  cases,  and  has  been  studied 
systematically  by  Townsend.  It  explains  the 
large  currents  which  can  be  carried  by  the  electric 
arc  or  spark  discharge. 

These  considerations  indicate  that,  in  the  experi- 
ments we  are  now  describing,  the  limit  of  the 
second  stage,  in  which  some  but  not  all  of  the 
negative  ions  are  stopped  by  the  magnetic  field, 
gives  the  distance  at  which  those  ions  coming 
from  the  surface  of  the  zinc  plate  just  fail  to  get 
across  the  space  between  the  plates.  The  expres- 
sion given  above  then  leads  directly  to  a  value  for 
e/m,  the  ratio  of  the  ionic  charge  to  the  ionic  mass. 
Thomson  found  as  the  result  7.3  x  io6,  a  number 
which  agrees  extremely  well  with  that  deduced  for 
cathode  rays,  namely,  7.7  x  io6. 

With  the  negative  ions  produced  by  the  inci- 
dence of  ultra-violet  light  on  a  zinc  plate,  it  is  easy 
to  repeat  C.  T.  R.  Wilson's  experiments  on  the 
formation  of  clouds  round  ions  as  nuclei,  and  thus 


i8o  PHYSICAL  SCIENCE 

to  determine  the  value  of  e}  the  electric  charge 
associated  with  the  same  ions  for  which  e/m  has 
already  been  obtained.  The  result  shows  that,  as 
always,  the  charge  is  the  same  as  the  charge  on  an 
ion  in  liquid  electrolytes  ;  and  therefore  for  the  ions 
due  to  ultra-violet  light,  as  for  the  cathode  ray 
particles,  the  mass  must  be  about  the  seven  or 
eight-hundredth  part  of  the  mass  of  the  hydrogen 
atom.  The  result  has  been  confirmed  by  Lenard, 
who  used  a  somewhat  different  type  of  apparatus. 
In  all  these  investigations  the  existence  of 
particles  much  smaller  than  the  smallest  of  the 
hitherto  indissoluble  chemical  atoms  is  clearly 
indicated.  Since  the  beginning  of  the  nineteenth 
century  the  chemical  atom  has  been  the  ultimate 
unit  in  which  our  conception  of  matter  has  been 
expressed.  The  sixty,  seventy,  or  eighty  different 
elements,  progressively  known  to  the  chemist, 
seemed  to  be  essentially  different  in  kind,  though 
certain  likenesses  between  them,  and  periodic 
relations  between  their  properties  and  masses, 
vaguely  pointed  to  a  common  origin.  Now, 
after  a  hundred  years  of  usefulness,  the  atom 
yields  place  to  Professor  Thomson's  corpuscle  ; 
while  the  new  phenomena  of  radio-activity,  as  we 
shall  see  hereafter,  have  shaken  the  belief  in  the 
immutability  of  the  elements,  and  are  leading  to 
a  new  faith  in  the  transmutation  of  matter. 


CONDUCTION  THROUGH  GASES       181 

Speculation,  it  is  true,  from  the  days  of  Demo- 
critus  to  those  of  Sir  William  Crookes,  has  been 
busy  with  imaginings  anent  ultimate  particles, 
which  should  be  common  to  all  types  of  matter, 
and  should  compose  the  different  elements  by 
differences  in  their  number  or  arrangement.  But 
Professor  Thomson  has  not  followed  the  facile 
and  barren  paths  of  speculation.  He  has  first 
found  the  particles,  and  has  weighed  and  timed 
them  before  theorising  on  their  origin  and  destiny. 

We  are  now  in  a  position  to  estimate  the  im- 
portance of  the  experiments  which  have  shown 
that  the  mass  of  the  corpuscle  is  independent  both 
of  the  nature  of  the  gas  in  which  it  is  found,  and 
also  of  the  material  of  the  electrode  used  in  pro- 
ducing it.  Not  only  must  we  conceive  atoms  to 
be  made  up  of  these  more  minute  particles,  but  it 
is  necessary  to  suppose  that  in  all  atoms,  what- 
ever be  their  nature,  these  particles  are  similar. 
The  dream  of  an  ultimate  particle,  common  to  all 
kinds  of  matter,  has  thus  at  length  come  true. 

Further  evidence  is  not  wanting  in  support  of 
this  hypothesis.  The  absorption  of  ordinary  light 
by  different  substances  bears  no  relation  to  the 
density  of  the  absorbing  medium.  Heavy  mate- 
rials like  iron  or  glass,  light  bodies  such  as  cork 
or  water,  may  be  either  opaque  or  transparent. 
On  the  other  hand,  in  the  absorption  of  cathode 


1 82  PHYSICAL  SCIENCE 

rays,  and  of  the  corresponding  rays  which,  as  we 
shall  see,  are  emitted  by  radio-active  bodies,  very 
different  phenomena  appear.  A  given  thickness  of 
any  material,  whether  gas,  liquid,  or  solid,  absorbs 
these  rays  simply  in  proportion  to  its  mass, 
entirely  independently  of  any  other  property. 
Throughout  an  enormous  range  of  density,  various 
substances,  including  air,  the  heavy  gas  sulphur 
dioxide,  paper,  glass,  silver,  and  gold,  possess 
absorption  coefficients  directly  proportional  to 
their  densities.  This  remarkable  relation  is  ex- 
plained at  once  by  the  theory  we  are  considering. 
If  each  atom  of  matter  be  composed  of  a  number 
of  corpuscles,  and  these  corpuscles  be  extremely 
minute  compared  with  the  atom  as  a  whole,  it  is 
clear  that  we  must  regard  an  atom  as  an  open 
structure  in  which  the  vacant  spaces  are  immense 
compared  with  the  size  of  the  tiny  particles  scat- 
tered throughout  the  atomic  system  under  the 
controlling  influence  of  their  mutual  forces.  A 
collection  of  such  atoms,  forming,  let  us  say,  a  thin 
plate  of  aluminium,  might  be  quite  impervious  to 
other  atoms  as  wholes — it  might,  in  fact,  be  quite 
air-tight.  The  flight  of  isolated  corpuscles  in  a 
cathode  ray,  however,  if  travelling  fast,  might  be 
able  to  penetrate  the  plate  here  and  there,  the 
corpuscles  finding  their  way  between  the  atoms,  or 
through  the  inter-corpuscular  spaces  in  the  struc- 


CONDUCTION  THROUGH  GASES       183 

ture  of  the  atoms  themselves.  The  corpuscles 
being  all  similar  to  each  other,  the  relative  densities 
of  two  substances,  such  as  aluminium  and  air, 
must  depend  simply  on  the  relative  numbers  of 
corpuscles  which  make  up  the  atoms  contained  in 
unit  volume  of  each  material.  The  power,  pos- 
sessed by  different  kinds  of  matter,  of  stopping 
cathode  rays,  will  also  simply  depend  on  the 
number  of  such  corpuscles  contained  in  unit 
volume  of  the  different  absorbing  media,  for  the 
particles  in  the  rays  will  readily  pass  through  the 
open  structure  of  the  atomic  systems,  but  will  be 
stopped  by  collision  with  the  substance  of  the 
individual  corpuscles  which  make  up  those 
systems.  The  absorbing  powers  must  thus,  on 
our  theory,  as  well  as  in  fact,  be  proportional  to 
the  density  of  the  material,  and  independent  of  all 
its  other  properties. 

The  relation  between  the  corpuscles  and  the 
electric  charges  associated  with  them  must  next 
be  considered.  These  particles  have  never  been 
observed  with  positive  charges  ;  positive  ions  are 
usually  found  to  have  masses  about  equal  to 
those  of  the  chemical  atoms.  The  facts  may  pro- 
visionally be  explained  by  the  hypothesis  that  the 
corpuscle  constitutes  the  isolated  negative  unit  of 
electricity,  of  which,  under  the  name  of  an  elec- 
tron, the  hypothetical  existence  and  properties 


1 84  PHYSICAL  SCIENCE 

have  been  studied  by  Professor  Larmor  and  other 
mathematicians.  An  atom  of  ordinary  matter, 
with  one  corpuscle  beyond  its  proper  number,  is 
an  atom  negatively  electrified  ;  an  atom  with  the 
corpuscle  detached  from  it  is  an  atom  positively 
electrified.  These  charged  atoms  act  as  ions, 
negative  and  positive  respectively,  in  accordance 
with  the  usual  convention  about  signs. 

Speculation  has  gone  even  farther  than  this. 
A  moving  electrified  body  acts  like  an  electric 
current,  and,  as  we  have  seen,  is  associated 
with  electro-magnetic  energy  and  electro-magnetic 
momentum  in  the  surrounding  dielectric  medium. 
To  change  the  velocity,  therefore,  requires  the  ex- 
penditure of  electro-magnetic  energy,  and  thus  the 
electrified  body  possesses  electric  inertia  in  addi- 
tion to  its  ordinary  dynamical  inertia.  As  long  as 
the  velocity  is  small,  this  electric  inertia  is  constant, 
but  an  electrified  body  moving  rapidly  can  be 
shown  mathematically  to  behave  as  though  its 
inertia,  that  is,  its  mass,  were  increased  ;  and,  as 
the  velocity  of  light  is  approached,  this  apparent 
electric  mass  grows  very  rapidly.  Some  recent 
experiments  by  Kaufmann,  in  which  the  masses  of 
the  negative  corpuscles  emitted  by  radium  were 
investigated,  are  of  intense  interest  in  this  con- 
nection. The  radium  corpuscles  move  much  more 
rapidly  than  those  found  in  cathode  rays,  though 


CONDUCTION  THROUGH  GASES       185 

in  other  respects  corpuscles  from  the  two  sources 
appear  to  be  identical.  With  radium  the  velo- 
cities are  so  great  that  they  closely  approach  that 
of  light.  A  speed  of  2.85  x  io10  centimetres  a 
second  has  been  observed,  that  of  light  itself  being 
only  3.0  x  io10.  At  these  enormous  velocities, 
Kaufmann  found  that  the  value  of  e/tn,  determined 
from  the  magnetic  and  electric  deflections,  was 
considerably  diminished,  a  value  of  0.63  x  io7 
being  obtained.  Assuming  that  the  charge  be 
constant,  this  means  an  increase  in  m,  the  effective 
mass  of  the  corpuscles. 

From  the  theory  of  electrons  it  is  possible  to 
calculate  what  the  increase  of  apparent  mass 
should  be,  on  the  assumption  that  the  whole  of 
the  mass  of  the  corpuscle  is  an  electrical  mani- 
festation, and,  as  we  shall  see  in  a  future  chapter, 
the  results  of  Kaufmann's  experiments  agree  with 
the  calculated  numbers.  Such  results  are  of 
fundamental  importance,  both  physically  and 
philosophically.  It  is  probable  that  the  whole  of 
the  observed  mass  of  the  corpuscle  is  in  reality 
an  effect  due  to  the  electro-magnetic  inertia  of  its 
electric  charge.  Representing  the  atoms  of  ordi- 
nary matter  as  made  up  of  corpuscles,  and  identi- 
fying the  corpuscles  with  electrons  or  isolated 
electric  charge-units,  it  becomes  possible  to  ex- 
plain their  mass  by  the  electro-magnetic  properties 


1 86  PHYSICAL  SCIENCE 

of  a  moving  charge.  To  explain  the  phenomena 
of  radiation,  it  is  necessary  to  suppose  that  the 
electrified  corpuscles — the  electrons — are  in  rapid 
orbital  or  oscillatory  motion  within  the  atom : 
that,  for  example,  the  electrons  whirl  round  in 
their  orbits  as  the  planets  swing  round  the  sun. 

Mass  or  inertia  is  the  most  permanent  and  char- 
acteristic property  of  matter,  and  having  explainec 
mass  as  due  to  electricity  in  motion,  the  physicisl 
may  well  ask  the  metaphysical  question :  has 
matter  any  objective  reality  ;  may  not  its  very 
essence  be  but  a  form  of  disembodied  energy  ? 
But  then  arises  the  further  problem  of  the  natun 
of  electricity,  and  the  mystery  remains  as  great  as 
ever,  although  driven  one  step  farther  back. 

Instead  of  thus  stating  matter  in  terms  of  elec- 
tricity, it  is  simpler,  and  perhaps  less  ambitious,  to 
express  electricity  in  terms  of  matter,  as  we  have 
done  above  in  saying  that  electrified  atoms  con- 
tain one  or  more  corpuscles  in  excess  or  defect  of 
their  normal  number.  Nevertheless,  the  electron 
theory  of  matter,  formerly  supported  on  mathe- 
matical grounds,  has  been  strengthened  greatly  by 
these  recent  developments  of  experimental  science. 
Moreover,  from  the  point  of  view  of  radio-activity, 
which  we  shall  consider  in  the  next  chapter,  that 
theory  is  of  supreme  importance,  for,  if  the  atom 
consists  of  electrons  in  rapid  orbital  motion,  it  is 


CONDUCTION  THROUGH  GASES       187 

possible  that  some  corpuscles  may  occasionally  fly 
away  from  their  orbits.  In  a  few  such  cases 
many  corpuscles  may  depart  from  an  atom  at 
once,  and  leave  the  residue  in  an  unstable  state,  in 
which  the  rearrangement  of  the  remaining  cor- 
puscles into  new  atoms  is  necessary  for  equilibrium. 
Thus  the  electron  theory  suggests  the  occasional 
instability  of  matter.  Now  the  occasional  insta- 
bility of  a  complex  chemical  atom,  and  its  dis- 
integration into  simpler  bodies,  as  we  shall  presently 
see,  is  the  probable,  perhaps  the  certain,  explanation 
of  the  phenomena  of  radio-activity. 

Among  the  various  agencies  enumerated  at  the 
beginning  of  this  chapter  for  the  production  of 
gaseous  ions,  special  interest  attaches  to  the  action 
of  incandescent  metals  and  carbon.  Elster  and 
Geitel,  Richardson,  H.  A.  Wilson,  and  others  have 
shown  that,  as  a  platinum  wire  is  gradually  heated, 
it  begins  to  emit  positive  ions  at  a  temperature 
corresponding  to  a  low  red  heat.  The  investi- 
gation of  the  influence  of  a  magnetic  force  shows 
that  these  ions  vary  in  size,  some  probably  being 
molecules  of  the  gas,  and  others  molecules  of  the 
metal  or  even  dust  disintegrated  from  its  surface. 
As  the  platinum  is  still  further  heated,  negative 
ions  also  come  off,  ultimately  in  large  excess.  In 
vacuo  the  negative  leak  from  platinum  and  carbon 
filaments  is  very  large — from  carbon  it  may  even 


1 88  PHYSICAL  SCIENCE 

amount  to  as  much  as  an  ampere  of  current  from 
each  square  centimetre  of  surface.  The  negative 
ions  are  then  of  sub-atomic  dimensions;  and  are 
identical  with  the  corpuscles  otherwise  obtained. 
H.  A.  Wilson  has  shown  that,  at  the  lower  tem- 
peratures at  which  the  negative  leak  occurs,  it  is 
very  largely  due  to  the  effect  of  hydrogen  absorbed 
in  the  platinum,  and  liberated  under  the  action  of 
the  heat.  At  the  highest  temperatures,  however, 
the  corpuscles  due  to  the  wire  itself  seem  to  be 
much  more  numerous  than  those  depending  on 
the  presence  of  hydrogen,  and  to  the  metal  itself 
we  must  then  look  for  their  source. 

The  emission  of  corpuscles  at  high  temperatures 
is  not  confined  to  solids.  Thomson  finds  that 
sodium  vapour  also  gives  off  a  large  supply,  and 
the  effect  seems  to  be  common  to  all  kinds  of 
matter  at  a  white  heat.  Carbon  is  particularly 
efficacious,  perhaps  because  it  can  be  raised  to  a 
higher  temperature  than  can  metals.  It  is  easy  to 
demonstrate  the  existence  of  a  measurable  current 
from  one  limb  of  the  carbon  filament  of  an  ordi- 
nary incandescent  lamp  to  an  insulated  plate 
placed  between  the  limbs. 

Owing  to  the  emission  of  corpuscles  by  an 
incandescent  wire  or  carbon  filament  along  which 
a  current  flows,  the  effective  current-carrying  area 
of  the  wire  is  increased.  In  vacuo  a  considerable 
fraction  of  the  current  might  pass  through  the 


CONDUCTION  THROUGH   GASES       189 

space  surrounding  the  wire,  which  must  become 
filled  with  corpuscles.  Although  in  gases  at  ordi- 
nary pressures  the  emission  of  corpuscles  is  less 
copious,  still,  ionization  will  occur  to  an  appreciable 
extent  just  round  the  wire,  and  a  part,  though 
perhaps  a  small  part,  of  the  current  will  pass 
along  outside  the  substance  of  the  wire. 

The  phenomena  we  are  now  considering  must 
have  an  important  bearing  on  cosmical  processes. 
The  photosphere  of  the  sun  contains  large  quan- 
tities of  glowing  carbon,  and  this  carbon  will  emit 
corpuscles  until  the  resultant  positive  charge  left 
on  the  sun  exerts  an  electro-static  force  great 
enough  to  prevent  further  emission.  In  this  way 
a  condition  of  equilibrium  would  be  reached. 
Any  local  elevation  of  temperature  would  then 
cause  a  stream  of  corpuscles  to  leave  the  sun  and 
pass  into  the  surrounding  space.  When  corpuscles 
pass  through  a  gas  with  high  velocity,  they  make  it 
luminous,  and  Arrhenius  has  explained  many  of 
the  periodic  peculiarities  of  the  Aurora  Borealis  by 
the  supposition  that  corpuscles  from  the  sun,  due 
either  to  incandescence  or  to  some  other  cause, 
stream  through  the  upper  regions  of  the  earth's 
atmosphere. 

The  phenomena  of  electrolytic  conduction 
through  liquids,  and  of  non-electrolytic  conduction 


I9o  PHYSICAL  SCIENCE 

through  metallic  substances,  must  now  be  inter- 
preted in  terms  of  this  corpuscular  theory.  The 
chemical  decomposition  of  electrolytic  solutions, 
which  we  have  described  in  Chapter  IV.,  indicates 
that  an  electric  transfer  through  such  liquids  in- 
volves a  movement  of  the  chemical  constituents 
of  the  substance  decomposed.  In  fact,  as  we  have 
seen,  that  movement  has  been  experimentally  de- 
monstrated, and  the  passage  of  the  ions  rendered 
visible.  We  must  suppose,  then,  that  the  cor- 
puscle forming  the  effective  negative  essence  of 
the  anion,  is,  in  liquid  electrolytes,  attached  to 
an  atom  of  matter.  This  atom  may  possibly  be 
associated  with  other  atoms  or  molecules  forming 
a  complex  ion,  but  the  point  is  that  the  isolated 
corpuscle  cannot  slip  from  one  atom  to  another, 
and  thus  carry  an  electric  current  through  the 
liquid  ;  the  corpuscle  cannot  move  without  a  cor- 
responding movement  of  matter — of  matter,  that 
is,  in  its  atomic  or  molecular  sense. 

The  motion  of  the  positive  ion  here  again 
involves  the  simultaneous  passage  of  a  particle 
of  matter  of  at  least  atomic  dimensions.  The 
positive  ion  consists  of  an  atom  of  the  electrolyte 
with  one  of  its  corpuscles  missing.  In  this  way, 
a  unit  of  negative  electricity  is  removed  from  it, 
that  is,  it  is  left  with  a  positive  charge.  Electricity, 
on  this  hypothesis,  is  one,  not  two  ;  the  two 


CONDUCTION  THROUGH  GASES      191 

so-called  opposite  electricities  being  perhaps  an 
excess  or  defect  of  a  single  thing.  Thus  the 
nomenclature  of  the  old  "  one  fluid  theory "  is 
still  appropriate,  the  word  fluid  being  understood 
strictly  in  a  "  Pickwickian  sense."  It  is  true  that 
the  meaning  of  the  conventional  names  positive 
and  negative  should  be  reversed  ;  it  is  the  old 
negative  electricity  which,  from  this  point  of  view, 
is  the  real  corpuscle,  and  a  positive  charge  means 
a  deficiency  in  the  proper  number  of  the  electrical 
units.  But  the  signs  conventionally  attached  to 
electric  charges  are  a  mere  matter  of  historical 
accident,  and  the  fact  that  fate  wrongly  allotted 
the  names  is  in  harmony  with  the  frequent  experi- 
ence of  us  all,  in  cases  where  two  alternatives  are 
possible,  and  a  free  choice  is  assured. 

In  metals  an  electric  current  flows  without 
chemical  change  in  the  substance  of  the  con- 
ductor, so  that,  in  this  case,  we  must  imagine  the 
corpuscles  to  be  freely  mobile.  They  pass  from 
atom  to  atom,  and  thus  carry  the  current  when 
an  electromotive  force  acts.  In  the  presence  or 
absence  of  such  a  force,  they  may  be  regarded  as 
existing  within  the  metal  in  a  state  resembling 
in  many  ways  the  state  of  a  gas  in  a  closed  vessel. 
Estimates  have  been  made  of  the  number  of 
corpuscles  present  in  a  given  volume  ;  of  the 
velocity  with  which  they  move  under  an  electric 


192  PHYSICAL  SCIENCE 

force  ;  and  of  their  mean  free  path  within  the 
metal,  that  is,  of  the  average  distance  a  corpuscle 
moves  between  its  collisions  with  other  corpuscles. 
As  we  have  seen,  when  the  metal  is  heated,  the 
corpuscles  begin  to  leave  it,  and  stream  away 
into  the  surrounding  space.  At  any  temperature 
equilibrium  is  set  up  between  the  corpuscles 
leaving  the  metal  owing  to  the  effect  of  tem- 
perature, and  those  drawn  back  again  by  the 
residual  positive  charge  on  the  metal.  We  may 
look  on  the  system  as  analogous  to  a  liquid  in 
equilibrium  with  its  own  vapour. 

In  the  last  chapter  we  saw  that  it  was  neces- 
sary clearly  to  distinguish  the  electric  current 
and  the  heating  effect  of  the  current  from  the 
flow  of  the  energy  by  which  the  current  was 
maintained.  The  energy  passes  through  the 
surrounding  medium,  through  the  luminiferous 
aether.  The  current  is  merely  the  line  along 
which  the  energy  of  the  aether  can  be  dissipated 
as  heat.  Faraday  and  Maxwell  showed  that  the 
medium  invoked  to  explain  the  phenomena  of 
light  was  also  competent  to  explain  electric  and 
magnetic  manifestations.  An  electric  force  is  a 
state  of  strain  in  the  aether,  and  the  immediate 
function  of  an  electric  machine  or  voltaic  battery 
is  to  set  up  such  a  state  of  strain.  If  the  poles  of 
the  battery  are  insulated  from  each  other,  the  state 


CONDUCTION  THROUGH  GASES      193 

of  strain  is  maintained,  the  poles  are  attracted 
towards  each  other  with  a  small  force,  but  noth- 
ing else  happens.  Faraday,  as  we  have  seen,  re- 
presented this  state  of  strain  by  drawing  lines 
of  force,  or  tubes  of  force,  which  map  out  the 
electric  field,  and  everywhere  follow  the  direction 
of  the  electric  force.  The  tubes  of  electric  force 
end  on  the  surfaces  of  conductors,  and  the  oppo- 
site ends  of  each  tube,  where  they  touch  the 
conductors,  are  the  physical  meaning  of  unit 
electric  charges  of  opposite  sign.  The  state  of 
strain  in  the  field  is  such  that  we  must  imagine 
the  tubes  of  force  as  tending  to  shorten  in  length 
and  to  push  each  other  apart  ;  and,  when  the 
poles  of  a  battery  are  disconnected,  the  tubes  of 
force  will  be  in  equilibrium  under  these  forces 
The  distribution  of  the  electric  tubes  will  then 
be  very  similar  to  that  of  the  magnetic  lines, 
made  visible  by  the  filings  shown  in  Fig.  29  on 
page  1 68. 

A  conducting  wire  must  be  regarded  as  a 
channel  along  which  the  free  ends  of  a  line 
or  tube  of  force  can  move,  and,  when  the 
poles  of  the  battery  are  connected  by  means  of 
a  wire,  the  tubes  of  force  in  the  surrounding 
air  run  their  opposite  ends  on  to  the  wire,  pull 
those  ends  towards  each  other,  and  shut  up. 
Other  tubes  are  then  pushed  into  the  wire  by 

N 


194  PHYSICAL  SCIENCE 

their  mutual  transverse  pressure,  and  are  oblit- 
erated in  turn.  The  tubes  of  force  in  the 
dielectric  field  are  thus  inclined  to  disappear, 
and  the  state  of  aethereal  strain  in  that  field 
tends  to  be  relieved.  Simultaneously,  however, 
the  battery  endeavours  to  reassert  the  original 
distribution  of  tubes,  and  once  more  to  set  up 
the  strain.  In  this  way  new  tubes  are  constantly 
forming  between  the  terminals  of  the  battery, 
and  are  as  constantly  pushed  into  the  connect- 
ing wire,  where  they  vanish.  When  the  connec- 
tion is  metallic,  it  is  only  the  negative  ends  of 
the  tubes,  attached  to  the  corpuscles,  that  move, 
the  positive  ends  remain  at  rest.  If,  on  the 
other  hand,  part  of  the  circuit  is  composed  of 
an  electrolyte,  in  that  part  the  positive  ends  of 
the  tubes  are  also  mobile.  Now  it  is  this 
continual  process  of  establishment  of  aethereal 
strain  by  a  battery,  and  the  compensating  pro- 
cess of  its  obliteration  along  a  conductor  that, 
according  to  the  views  of  Faraday  and  Maxwell, 
now  universally  adopted,  constitute  an  electric 
current. 

The  ionic  theory  of  electrolysis  gave  a  clear 
idea  of  the  mechanism  by  which  the  slipping  of 
the  ends  of  the  tubes  of  force  occurred  in  con- 
ducting liquids,  and  now  the  corpuscular  hypo- 
thesis gives  us  an  equally  vivid  insight  into  the 


CONDUCTION  THROUGH  GASES      195 

nature  of  the  process  within  metallic  circuits. 
The  tubes,  anchored  by  their  ends  to  an  ion 
in  electrolytes  or  to  a  corpuscle  in  metals,  drag 
their  anchors.  It  is  the  slip  of  the  anchors 
that  constitutes  the  current,  and  the  heat  de- 
veloped by  the  passage  of  the  current  is  to  be 
explained  by  the  frictional  resistance  to  the  drag 
of  the  anchor,  or  to  some  other  means  of  dis- 
sipating energy,  such  as  inter-corpuscular  radia- 
tion, not  yet  fully  understood. 

Faraday  had  no  skill  in  mathematical  analysis, 
and  his  insight  into  physical  principles  is  one  of 
the  best  examples  of  scientific  instinct  found  in 
history.  As  was  well  said  by  Von  Helmholtz  in 
the  Faraday  Lecture  for  the  year  1881,  "  Now 
that  the  mathematical  interpretation  of  Faraday's 
conceptions  regarding  the  nature  of  electric  and 
magnetic  forces  has  been  given  by  Clerk  Maxwell, 
we  see  how  great  a  degree  of  exactness  and  pre- 
cision was  really  hidden  behind  the  words,  which 
to  Faraday's  contemporaries  appeared  either  vague 
or  obscure  ;  and  it  is  in  the  highest  degree  as- 
tonishing to  see  what  a  large  number  of  general 
theorems,  the  mathematical  deduction  of  which 
requires  the  highest  powers  of  mathematical 
analysis,  he  formed  by  a  kind  of  intuition,  with 
the  security  of  instinct,  without  the  help  of  a 
single  mathematical  formula.  I  have  no  intention 


196  PHYSICAL  SCIENCE 

of  blaming  his  contemporaries,  for  I  confess  that 
many  times  I  have  myself  sat  hopelessly  looking 
upon  some  paragraph  of  Faraday's  descriptions  of 
lines  of  force,  or  of  the  galvanic  current  being 
an  axis  of  power." 

Such  a  confession  from  a  man  of  the  com- 
manding ability  of  Von  Helmholtz  shows  how  far 
the  instinctive  genius  of  Faraday  had  carried  him 
in  advance  of  his  age.  "  We  must  also  in  his 
case  acquiesce  in  the  fact  that  the  greatest  bene- 
factors of  mankind  usually  do  not  obtain  a  full 
reward  during  their  lifetime,  and  that  new  ideas 
need  the  more  time  for  gaining  general  assent 
the  more  really  original  they  are,  and  the  more 
power  they  have  to  change  the  broad  path  of 
human  knowledge." 


CHAPTER     VI 

RADIO-ACTIVITY 

"  To  watch  the  abysm-birth  of  elements." 

— KEATS,  Endymion. 

SCIENTIFIC  investigation,  which  usually  proceeds 
unmarked  by  most  of  those  not  directly  engaged 
in  it,  is  from  time  to  time  forced  on  the  attention 
of  the  public  by  some  discovery  of  immediate  and 
striking  advantage  to  mankind,  or  by  the  attain- 
ment of  some  theoretical  result,  which,  from  its 
novelty  and  interest,  fires  the  imagination  of  every 
thinking  man. 

To  those  who  follow  closely  the  course  of 
research,  these  brilliant  advances  in  knowledge 
rarely  come  suddenly.  The  slow  and  patient  work 
of  many  observers  through  long  years  often  leads 
up  to  and  suggests  the  particular  step  from  which 
follows,  almost  of  necessity,  the  practical  appli- 
cation or  the  far-reaching  theory.  The  mathe- 
matical genius  of  Clerk  Maxwell,  the  experimental 
skill  of  Hertz,  laid  the  foundations  on  which,  some 
years  afterwards,  was  reared  the  superstructure  of 
wireless  telegraphy.  The  observations  of  Crookes, 


198  PHYSICAL  SCIENCE 

Lenard,  J.  J.  Thomson,  and  many  others,  on  elec- 
tric discharges  through  rarified  gases,  had  given 
to  the  physicist  an  extended  insight  into  the  nature 
of  these  phenomena,  before  Rontgen's  almost  acci- 
dental discovery — that  photographically  active  rays 
thus  obtained  could  traverse  certain  substances 
opaque  to  light — revealed  the  bones  in  his  hand 
to  the  man  in  the  street. 

General  attention  has  lately  been  directed  to 
the  subject  of  radio-activity.  M.  Curie  demon- 
strated that  the  stream  of  energy  constantly  pro- 
ceeding from  the  newly-discovered  element  radium 
could  be  detected  by  a  measurable  rise  of  tem- 
perature in  a  small  quantity  of  the  substance 
protected  from  loss  of  heat  ;  and  the  publication 
of  this  result  was  followed  by  a  correspondence  in 
the  Times,  in  which  some  surprising  efforts  were 
made  to  explain  the  source  of  the  energy,  and  to 
elucidate  the  "  mystery  of  radium." 

In  this  case  also  the  essential  phenomena  have 
been  under  investigation  longer  than  is  generally 
known  ;  and  their  detection  naturally  arose  from 
a  knowledge  of  the  properties  of  Rontgen  rays. 
These  rays  produce  fluorescent  effects  on  suitable 
screens  ;  and  it  was  natural  to  examine  phos- 
phorescent and  fluorescent  substances,  to  determine 
if  they  were  the  source  of  similar  radiation.  For 
some  time  no  definite  results  were  obtained  ;  but, 


RADIO-ACTIVITY  199 

in  the  year  1896,  M.  Henri  Becquerel  discovered 
that  compounds  of  the  metal  uranium,  whether 
phosphorescent  or  not,  affected  a  photographic 
plate  through  an  opaque  covering  of  black  paper, 
and  rendered  the  air  in  their  neighbourhood  a 
conductor  of  electricity. 

Such  were  the  first  observations  on  the  property 
of  radio-activity  ;  but  the  rapid  development  of 
the  subject  which  has  followed  could  only  have 
taken  place  with  the  aid  of  our  previous  knowledge 
of  the  electrical  properties  of  gases.  Although  the 
superficial  similarity  between  Becquerel  rays  and 
Rontgen  rays  has  proved  for  the  most  part  mis- 
leading, the  relations  between  the  two  branches  of 
the  subject  are  so  intimate  that  it  is  impossible  to 
study  satisfactorily  the  phenomena  of  radio-activity 
without  a  knowledge  of  the  results  previously  and 
simultaneously  reached  by  the  investigation  of 
electric  discharge  through  gases. 

After  Becquerel's  discovery  of  the  photographic 
and  electric  activity  of  uranium,  it  was  found 
that,  like  Rontgen  rays,  the  rays  from  uranium 
produced  electric  conductivity  in  air  and  other 
gases  through  which  they  passed.  In  the  year 
1900,  M.  and  Mme.  Curie  made  a  systematic 
search  for  similar  properties  in  a  great  number 
of  chemical  elements  and  compounds,  and  in 


200  PHYSICAL  SCIENCE 

many  natural  minerals.  They  found  that  several 
minerals  containing  uranium  were  more  active 
than  that  metal  itself.  Pitch-blende,  for  instance, 
a  substance  consisting  chiefly  of  an  oxide  of 
uranium,  but  containing  also  traces  of  many 
other  metals,  was  especially  active.  When  ob- 
tained from  Cornwall  its  activity  was  about  equal 
to  that  of  the  same  weight  of  uranium,  but 
samples  from  the  Austrian  mines  were  found  to  be 
three  or  four  times  as  effective.  The  presence  of 
some  more  active  constituent  was  thus  suggested. 
To  examine  this  point,  the  various  components  of 
pitch-blende  were  separated  chemically  from  each 
other  and  their  radio-activities  determined.  In 
this  way  three  different  substances,  radium,  polo- 
nium, and  actinium,  all  previously  unknown,  appear 
to  have  been  isolated  by  different  observers.  Of 
these  three  the  most  active  is  the  now  well-known 
radium,  discovered  by  M.  and  Mme.  Curie,  work- 
ing with  M.  B£mont. 

Radium  is  obtained  from  pitch-blende  in  com- 
pany with  the  metal  barium  ;  and  the  two  seemed 
at  first  to  be  connected  chemically  so  intimately 
that  the  new  substance  was  for  a  time  called 
"  active  barium."  However,  a  slight  difference  in 
the  solubilities  of  some  of  their  salts  enables  them 
to  be  gradually  separated  by  a  process  of  repeated 
fractionisation,  the  radium  chloride  an4  bromide 


RADIO-ACTIVITY  201 

crystallising  out  more  readily  than  the  correspond- 
ing compounds  of  barium. 

These  processes  of  chemical  separation  are 
remarkable  for  their  use  of  the  new  property  of 
radio-activity  as  a  sole  guide  in  the  operations. 
After  each  reaction  the  activities  of  both  the  pro- 
duct and  the  residue  were  determined.  It  was  thus 
settled  whether  the  reaction  just  tried  was  effective, 
and  in  which  of  the  substances  separated  by  the 
reaction  the  property  of  radio-activity  had  been 
concentrated. 

The  quantity  of  radium  present  in  pitch-blende 
is  extremely  small,  many  tons  of  the  mineral 
yielding,  after  long  and  tedious  work,  only  a  small 
fraction  of  a  gramme  of  an  impure  salt  of  radium. 
Its  extraction  is  consequently  a  matter  of  great 
labour  and  high  cost.  Radium  salts  of  fair  purity 
have  now  become  articles  of  commerce,  at  all 
events  in  Germany,  though  the  supply  is  usually 
insufficient  to  meet  the  demand ;  and  radium  is 
at  present  worth  many  thousand  times  its  weight 
in  gold. 

An  interesting  point  in  these  investigations  is 
the  extreme  sensitiveness  of  the  property  of  radio- 
activity as  a  test  for  the  presence  of  those  sub- 
stances which  possess  it.  A  delicate  electroscope 
will  easily  show  a  leak  of  electricity  with  a 
substance  having  an  activity  of  about  the  one 


202  PHYSICAL  SCIENCE 

hundredth  part  of  that  possessed  by  uranium. 
The  activity  of  pure  radium  has  been  estimated 
as  about  one  million  times  that  of  uranium  ;  and 
such  radium  is  a  definite,  well-marked  chemical 
element,  like  other  elements,  forming  salts  and 
other  chemical  compounds,  and  giving  strong 
bright  lines  when  heated  and  examined  with  a 
spectroscope.  Spectrum  analysis  has  hitherto 
been  the  most  delicate  means  at  our  disposal  for 
detecting  the  presence  of  the  chemical  elements  ; 
but  in  the  preparation  of  radium  from  pitch-blende 
its  spectrum  only  began  to  appear  when,  in  the 
prolonged  process  of  fractionisation,  the  product 
had  reached  an  activity  of  about  fifty  times  that  of 
uranium. 

It  appears  from  these  figures  that  the  electro- 
scopic  method  of  detecting  radio-active  matter  is 
several  thousand  times  more  sensitive  than  the 
most  refined  methods  of  spectrum  analysis,  and  in 
other  cases  a  still  greater  sensitiveness  seems  to 
have  been  reached.  History  has  again  repeated 
itself.  When  the  spectroscope  was  first  placed  in 
the  hands  of  chemists,  it  revealed  the  existence  of 
several  elements  which  occurred  in  quantities  too 
small  to  be  detected  by  any  other  means  then 
known.  In  a  similar  way  additional  elements  have 
now  been  detected  and  isolated  by  the  help  of  the 
newer  and  more  powerful  method  of  research, 


RADIO-ACTIVITY  203 

In  the  year  1899  Professor  Rutherford  of  Mon- 
treal, one  of  the  band  of  physicists  trained  by 
Professor  J.  J.  Thomson  at  Cambridge,  discovered 
that  the  radiation  from  uranium  consists  of  two 
distinct  parts.  One  part  was  found  to  be  unable 
to  pass  through  more  than  about  four  layers  of 
thin  aluminium  foil,  while  the  other  part  would 
pass  through  about  one  hundred  layers  before  its 
intensity  was  reduced  by  one  half.  The  first 
named,  or  a  rays,  produce  the  most  marked 
electric  effects,  while  the  more  penetrating,  or  /3 
rays,  are  those  which  affect  a  photographic  plate 
through  opaque  screens.  At  a  later  date  was 
detected  a  third  type  of  still  more  penetrating 
radiation,  known  as  j  rays,  which  can  traverse 
plates  of  lead  a  centimetre  thick,  and  still  produce 
photographs  and  discharge  electroscopes.  In 
proportion  to  its  general  activity,  radium  evolves 
all  three  types  of  radiation  much  more  freely 
than  uranium,  and  is  best  employed  for  their 
investigation. 

The  moderately  penetrating,  or  /3  rays,  can 
easily  be  deflected  by  a  magnet ;  and  Becquerel, 
who  deflected  them  by  an  electric  field  as  well, 
conclusively  proved  that  they  were  projected  par- 
ticles, charged  with  electricity.  M.  and  Mme. 
Curie  had  shown  previously  by  direct  experiment 
the  existence  of  a  negative  charge  associated  with 


204  PHYSICAL  SCIENCE 

these  rays.  Owing  to  their  ionizing  action,  it  is 
impossible  to  demonstrate  that  a  body  surrounded 
by  air  gains  a  charge  when  exposed  to  the  rays. 
Such  a  charge  would  leak  away  as  fast  as  it 
was  acquired.  But  by  working  in  a  very  good 
vacuum,  or  by  surrounding  the  body  with  a  solid 
dielectric  such  as  paraffin,  the  acquisition  of  a 
negative  charge  can  be  demonstrated  by  means 
of  an  electrometer.  Further  investigation  showed 
that  the  ft  rays  behave  in  all  respects  like  cathode 
rays,  although  they  possess  greater  velocities  than 
any  cathode  rays  hitherto  examined,  velocities 
which  have  different  values  ranging  from  60  to 
95  per  cent,  of  the  velocity  of  light.  The  ft 
rays,  then,  are  negative  corpuscles,  or  negative 
electrons. 

Magnetic  and  electric  fields  which  are  strong 
enough  to  deflect  considerably  the  ft  rays,  produce 
no  effect  on  the  easily  absorbed  a  rays.  Although 
Strutt,  in  the  year  1900,  had  suggested  that  the 
a  rays  were  positively  charged  particles,  of  mass 
greater  than  that  of  the  particles  which  constitute 
the  negative  ft  rays,  it  was  not  till  some  time  after- 
wards that  their  magnetic  and  electric  deviations 
were  experimentally  demonstrated,  and  shown  to 
be  in  the  direction  opposite  to  that  observed  with 
ft  rays.  The  mass  of  the  carriers  in  the  a  rays 
appeared  to  be  about  that  of  helium  atoms — more 


RADIO-ACTIVITY  205 

than  one  thousand  times  greater  than  that  of  the 
negative  corpuscles — but  evidence  is  now  accumu- 
lating which  casts  some  doubt  on  this  result.  The 
velocity  is  about  one-tenth  of  that  of  light.  The 
very  penetrating  or  y  rays  have  never  been  de- 
flected, and  from  this  evidence  it  has  been  supposed 
that  they  are  different  in  kind  to  the  other 
types,  and,  like  the  X  rays  discovered  by  Ront- 
gen,  consist  of  wave-pulses  travelling  through 
the  aether  with  the  velocity  of  light.  On  the 
analogy  of  the  cathode  rays,  we  should  expect 
that  such  pulses  would  be  started  as  a  secondary 
effect  of  the  /3  rays  ;  but,  in  August  1903,  Strutt 
published  experiments  which  show  that,  as  with 
the  a  and  /3  rays,  and  also  with  the  cathode 
rays,  different  gases  absorb  the  7  rays  in  direct 
proportion  to  the  density.  Such  results  were 
in  favour  of  the  view  which  regards  the  y  rays 
as  particles  of  some  kind  travelling  at  speeds 
exceeding  those  of  the  other  rays,  and  perhaps 
carrying  no  electric  charges,  for  the  absorption 
phenomena  exhibited  by  ordinary  Rontgen  rays 
are  of  an  entirely  different  kind.  But  further 
evidence  has  just  appeared.  In  March  1904  it 
was  announced  by  Mr.  A.  S.  Eve  and  by  Mr.  R.  K. 
M 'Clung  that  very  "hard"  Rontgen  rays — that 
is,  the  thin  and  intense  electro-magnetic  pulses 
produced  by  the  cathode  rays  of  very  high 


206  PHYSICAL  SCIENCE 

vacua — show  absorption  phenomena  similar  to 
those  of  the  y  rays  of  radium.  Forasmuch,  as 
the  ft  rays  travel  with  velocities  higher  than 
those  of  any  ordinary  cathode  rays,  we  should 
naturally  expect  the  resulting  pulses  to  be  thinner 
and  more  intense  than  ordinary  Rontgen  rays. 
It  seems  clear,  therefore,  that  y  rays  are  identi- 
cal in  origin  and  character  with  very  "hard" 
Rontgen  rays. 

All  the  three  types  of  radiation,  when  they 
pass  through  air  or  any  other  gas,  render  the 
gas  a  conductor  of  electricity,  so  that  the  charge 
of  an  electroscope  or  of  an  electrometer  leaks 
away.  The  charged  particles  of  doubtful  mass 
which  constitute  the  a  rays,  the  negative  cor- 
puscles or  electrons  which  form  the  ft  rays,  and 
the  intense  electro-magnetic  pulses  of  the  y  rays 
are  all  able  to  convert  some  of  the  molecules  of  a 
gas  into  electrified  ions.  The  a  and  ft  projectiles 
probably  effect  this  change  by  the  energy  of  their 
collisions  with  the  molecules  of  gas,  and  it  is 
possible  to  estimate  the  number  of  ions  produced 
by  each  shot.  It  has  been  reckoned  that  this 
number  is  sufficient  to  give  air  a  measurable 
conductivity  when  one  positive  particle  per  second 
is  emitted  by  the  radio-active  substance.  Even 
if  one  atom  of  radium  emits  only  one  such  par- 
ticle, this  estimate  means  that  the  electroscope  is 


RADIO-ACTIVITY  207 

able  to  detect  effects  which  depend  on  one  atom 
coming  into  action  each  second.  We  may  well 
be  astonished  at  the  delicacy  of  this  means  of 
research. 

Again,  all  three  kinds  of  rays  produce  phos- 
phorescent and  photographic  effects,  though  the 
penetrating  power  of  the  ft  and  y  rays  makes 
the  phenomena  due  to  them  more  remarkable. 

Radium  salts  are  self-luminous,  owing  either  to 
the  direct  emission  of  light  by  their  agitated  atoms, 
or  to  some  phosphorescent  effect  of  the  internal 
bombardment  produced  by  their  radio-activity. 
The  spectrum  of  this  spontaneous  luminosity  has 
been  photographed  by  Sir  William  and  Lady 
Huggins,  and  was  shown  to  correspond  with  the 
spectrum  obtained  by  passing  electric  sparks 
through  nitrogen.  Sir  William  Crookes  and  Pro- 
fessor Dewar  found  that  this  spectrum  vanished 
when  the  radium  compound  was  placed  in  a 
high  vacuum.  Probably,  therefore,  it  is  due  to  the 
effect  of  the  activity  of  the  radium  on  atmospheric 
nitrogen  surrounding  the  radium  salt  or  occluded 
within  it. 

A  screen  of  the  phosphorescent  substance,  zinc 
sulphide,  when  placed  in  the  neighbourhood  of  a 
radium  compound,  glows  brightly,  and  Crookes 
has  used  this  property  in  a  most  striking  and 
beautiful  experiment.  A  tiny  fragment  of  a  radium 


2o8  PHYSICAL  SCIENCE 

salt  is  fixed  at  the  distance  of  a  fraction  of  a  milli- 
metre in  front  of  a  plate  covered  with  zinc  sul- 
phide. On  looking  through  a  lens  or  a  low-power 
microscope  in  a  dark  room,  brilliant  scintilla- 
tions are  seen,  and  the  splash  of  the  atomic 
projectiles  of  the  a  radiation  as  they  strike  the 
target  is  thus  made  visible  to  the  human  eye. 
It  seems  probable  that  each  scintillation  is  im- 
mediately due  to  a  cleavage  of  one  of  the  tiny 
crystals  of  zinc  sulphide  under  the  influence  of 
the  bombardment. 

In  the  year  1900  Rutherford  made  another 
striking  discovery.  The  radiation  from  thorium 
was  known  to  be  very  capricious,  being  affected 
especially  by  slight  currents  of  air  passing  over  the 
surface  of  the  active  material.  Rutherford  traced 
this  effect  to  the  emission  of  a  substance  which 
behaved  like  a  heavy  gas  having  temporary 
radio-active  properties.  This  emanation,  as  it 
was  named,  is  to  be  distinguished  clearly  from 
the  radiations  previously  described,  which  travel 
in  straight  lines  with  velocities  approaching  that 
of  light.  The  emanation  diffuses  slowly  through 
the  atmosphere,  as  would  the  vapour  of  a  vola- 
tile liquid.  It  acts  as  an  independent  source  of 
straight  line  radiations,  but  suffers  a  decay  of 
activity  with  time. 

Similar    emanations    are   given   off   by    radium 


RADIO-ACTIVITY  209 

and  actinium,  but  not  by  polonium  or  uranium. 
The  emanations  seem  to  be  very  inert  chemi- 
cally, in  this  resembling  gases  of  the  argon  group. 
They  pass  unchanged  through  acids  or  hot 
tubes,  but  are  condensed  at  the  temperature  of 
liquid  air,  evaporating  again  as  the  tube  is 
warmed.  By  taking  advantage  of  this  property 
many  pretty  lecture-room  experiments  may  be 
performed.  For  example,  a  quantity  of  radium 
emanation  is  condensed  in  a  tube  surrounded 
with  liquid  air.  The  tube  is  connected  with 
others,  and,  if  the  liquid  air  be  removed,  the 
emanation  can  be  traced  as  it  diffuses,  by  the 
fluorescence  it  excites  on  the  glass,  or  on  small 
pieces  of  paper  covered  with  zinc  sulphide,  which 
are  placed  here  and  there  within  the  tubes.  By 
measuring  the  rates  of  diffusion  of  the  emana- 
tions into  other  gases,  their  densities  have  been 
approximately  determined  and  found  to  be  about 
one  hundred  times  that  of  hydrogen. 

When  the  emanations  come  into  contact  with 
solid  bodies,  they  cause  these  bodies  themselves 
to  become  temporarily  radio-active.  This  excited 
or  induced  radio-activity,  which  is  found  to  be 
acquired  more  readily  by  negatively  electrified 
surfaces,  is  apparently  due  to  radio-active  posi- 
tively electrified  particles  clinging  to  the  surfaces. 
Whatever  the  effective  substance  may  be,  it,  or 

O 


210  PHYSICAL  SCIENCE 

the  matrix  in  which  it  is  deposited,  can  be  dis- 
solved in  some  acids  and  regained  as  a  radio- 
active residue  on  evaporation. 

All  the  three  types  of  radiation  considered 
above,  and  known  as  a,  /3,  j  rays,  have  one 
remarkable  property  which,  at  first  sight,  is  not 
shared  by  the  emanations  just  described.  The 
radio-activity  of  any  element,  with  regard  to  the 
emission  of  these  rays,  is  independent  of  the 
compound  in  which  that  element  is  contained. 
Thus,  for  a  mass  containing  the  same  amount 
of  the  element  radium,  the  activity  of  radium 
chloride  is  the  same  as  that  of  radium  bromide; 
while  uranium,  the  metal,  has  the  same  activity  as 
it  has  when  chemically  combined  in  uranium 
nitrate.  Moreover,  an  alteration  in  the  physical 
conditions,  such  as  temperature,  which  always 
largely  influence  the  course  of  ordinary  physical 
and  chemical  changes,  seems,  throughout  an  ex- 
tended range,  to  be  entirely  without  effect  on 
the  processes  involved  in  radio-activity.  Heat- 
ing to  redness,  or  exposure  to  the  extreme  cold 
of  liquid  air,  equally  leave  the  activities  we  are 
considering  untouched.  Pursuing  these  investiga- 
tions to  even  lower  temperatures,  M.  Curie, 
during  a  visit  to  England  in  the  summer  of 
1903,  took  advantage  of  the  resources  of  the 
Royal  Institution  to  examine  the  properties  of 


RADIO-ACTIVITY  211 

radium  when  exposed  to  the  temperature  of 
liquid  hydrogen.  Professor  Dewar's  calorimeters 
then  indicated  that  the  heat  produced  by  0.7 
gramme  of  a  salt  of  radium  was  somewhat 
greater  than  at  higher  temperatures;  at  any 
rate,  it  was  not  less.  In  liquid  hydrogen  most 
chemical  activities  are  entirely  suspended,  and 
the  result  obtained  by  M.  Curie  and  Professor 
Dewar,  to  whatever  cause  it  may  be  due,  is 
very  remarkable.  Whether  or  not  the  increase 
they  then  noted  be  confirmed  by  further  experi- 
ments, it  seems  certain  that,  till  we  thus  ap- 
proach the  absolute  zero,  all  the  activities  of 
radium  are  quite  independent  of  temperature. 
Such  extraordinary  results  as  these  point  to  a 
deep-seated  difference  in  kind  between  the  radio- 
active processes  and  all  chemical  and  physical 
operations  hitherto  investigated.  We  shall  pre- 
sently examine  this  point  more  closely. 

Unlike  the  "  straight  line "  radiations  of  the 
types  a,  /3,  and  <y,  the  emanations  discovered  by 
Professor  Rutherford  are  emitted  much  more 
freely  from  some  compounds  of  the  radio-active 
element  than  from  others,  while  the  rate  of 
emission  is  largely  dependent  on  physical  condi- 
tions, such  as  the  temperature  of  the  system. 
By  a  striking  series  of  experiments,  however, 
Rutherford  has  traced  these  differences  to  varia- 


212  PHYSICAL  SCIENCE 

tions  in  the  ease  with  which,  after  formation,  the 
emanation  escapes  from  the  generating  substance. 

Let  us  consider  these  results  in  more  detail. 
It  is  found,  for  example,  that  while  the  emanation 
is  given  off  very  slowly  from  dry  and  solid  radium 
chloride,  it  is  freely  emitted  from  the  same  salt 
in  solution.  This  allows  the  problem  to  be  sub- 
mitted to  the  test  of  quantitative  experiment.  The 
rate  of  decay  of  the  radium  emanation  is  known  ; 
its  activity  falls  to  half  value  in  3.7  days.  Thus, 
the  activity  of  the  emanation  stored  in  a  solid 
radium  salt  reaches  a  limit,  when  its  rate  of 
decay  becomes  equal  to  the  constant  rate  at 
which  the  emanation  is  produced  by  the  radium. 
On  the  hypotheses  that  the  emanation  is  formed 
at  the  same  rate  in  the  solid  as  in  the  solution, 
that  it  escapes  from  the  solution  as  fast  as  it  is 
formed,  and  that  it  does  not  appreciably  escape 
from  the  solid  at  all,  it  is  clearly  possible  to 
calculate  the  amount  of  emanation  that  should  be 
stored  in  the  solid,  as  compared  with  the  amount 
produced  and  emitted  by  the  solution  in  a  given 
time. 

The  calculation  shows  that  463,000  times  more 
should  be  stored  in  the  solid  than  is  emitted  by 
the  solution  in  one  second.  Now  if,  as  supposed, 
the  emanation  is  stored  in  the  solid,  this  large 
amount  will  be  liberated  instantaneously  when 


RADIO-ACTIVITY  213 

that  solid  is  dissolved  in  water.  Rutherford 
and  Soddy  measured  this  rush  of  emanation  by 
its  effect  on  an  electroscope,  and  found  that  it 
was  477,000  times  greater  than  the  quantity 
afterwards  developed  by  the  solution  in  one 
second  :  a  remarkable  confirmation  of  the  several 
hypotheses  given  above. 

The  effect  of  raising  the  temperature  is  similar 
to  that  of  solution.  When  a  solid  radium  com- 
pound is  brought  to  a  red  heat,  a  rush  of  emana- 
tion takes  place,  which  makes  the  initial  emanating 
power  some  hundred  thousand  times  greater  than 
that  of  the  cold  solid.  This  high  rate  of  emis- 
sion, however,  does  not  last  ;  it,  also,  is  due  to 
the  rapid  escape  of  stored  material. 

By  experiments  such  as  these,  the  emanating 
power  of  radio-active  elements  has  been  brought 
into  line  with  their  other  radio-active  properties, 
and  has  been  shown  to  depend  only  on  the  mass 
of  the  element  present,  whatever  be  the  state  of 
combination  in  which  that  element  exists,  and 
whatever  be  the  physical  conditions  under  which 
the  process  occurs. 

Soon  after  appreciable  quantities  of  radium 
were  available  for  investigation,  Giesel  drew  atten- 
tion to  the  fact  that  a  radium  compound  gradu- 
ally increases  in  activity  after  formation,  and 


214  PHYSICAL  SCIENCE 

only  reaches  a  constant  state  after  a  month's 
interval.  Similar  phenomena  have  been  observed 
by  Curie  and  Dewar  for  the  heat  effect.  These 
results  are  readily  explained  if  we  consider  the 
properties  of  the  emanation  as  elucidated  by  the 
experimental  evidence  that  has  now  accumulated. 

When  a  salt  of  radium  is  dissolved  in  water, 
and  the  solution  boiled,  the  emanation  previously 
stored  in  the  salt  is  evolved  and  removed.  The 
residual  activity  of  the  salt  is  then  found  to  be 
much  diminished.  This  activity  must  include  that 
due  to  the  radium  itself,  and  also  the  excited 
activity,  which  has  been  developed  by  the  emana- 
tion, but  is  not  removed  with  it.  The  effect 
of  the  excited  activity  decays  rapidly ;  after  a 
few  hours  it  will  nearly  have  vanished,  and 
we  then  get  the  true  activity  of  the  pure 
radium  salt  alone,  uncomplicated  by  that  of  the 
emanation,  or  by  the  excited  activity  which  is 
produced  by  the  emanation. 

This  residual,  non-separable  activity  is  found 
to  consist  entirely  of  a  rays,  and,  measured 
electrically,  is  about  25  per  cent,  of  the  normal 
activity  of  a  radium  compound  after  a  month's 
existence  ;  a  normal  activity  which  comprises 
the  combined  effects  of  radium,  of  the  radium 
emanation,  and  of  the  excited  activity. 

Rutherford   and    Soddy  studied   these  relations 


RADIO-ACTIVITY  215 

in  detail.  They  dissolved  a  radium  compound, 
removed  the  emanation,  and  waited  till  the  ex- 
cited activity  had  subsided.  The  solution  was 
then  evaporated,  and  the  recovery  of  the  activity  of 
the  solid  crystals  of  salt  was  traced  by  measuring 
at  intervals  the  ionizing  power.  The  results  are 
shown  in  Fig.  32,  where,  neglecting  the  residual 


100 


16 

FIG.  32. 

activity,  the  recovery  curve  of  the  activity  of  the 
salt  is  compared  with  the  curve  of  decay  of 
activity  of  the  separated  emanation.  It  will  be 
seen  that  the  two  curves  are  complementary  to 
each  other  ;  the  activity  of  the  emanation  falls 
to  half  its  initial  value  in  a  little  less  than  four 
days,  and  the  purified  radium  salt  recovers  half 
its  final  activity  in  the  same  time.  If  the  ac- 


216  PHYSICAL  SCIENCE 

tivity  of  the  emanations  at  any  instant  be  added 
to  that  of  the  recovering  radium,  the  result  is 
equal  to  the  normal  activity  of  the  radium  when 
fully  recovered.  Thus  the  total  activity  of  the 
residual  radium  and  its  separated  emanation, 
considered  together,  remains  constant  through- 
out, though  resolved  into  constituent  portions. 
This  result  again  illustrates  the  characteristic 
feature  of  radio-active  processes :  the  impos- 
sibility of  changing  the  amount  of  activity  by 
any  known  chemical  or  physical  operations. 

Since  the  phenomena  of  radio-activity  have 
been  well  known,  and  the  various  types  of 
radiation  and  emanation  which  proceed  from  radio- 
active materials  clearly  distinguished,  traces  of  the 
property  have  been  found  to  be  disseminated  very 
widely.  Mr  C.  T.  R.  Wilson,  for  example,  has 
detected  radio-activity  in  newly-fallen  rain  and 
snow ;  when  evaporated  they  leave  a  residue 
which  discharges  an  electroscope.  Professor  J.  J. 
Thomson  has  found  that  when  air  is  bubbled 
through  various  samples  of  water  from  deep 
wells,  or  when  the  water  is  boiled  and  the  dis- 
solved air  driven  off  and  collected,  there  is 
present  in  the  air  a  radio-active  gas,  which 
behaves  as  though  it  were  the  emanation  from  some 
active  substance  of  which  slight  traces  are  con- 


RADIO-ACTIVITY  217 

tained  in  the  water.  The  air  loses  its  active  pro- 
perties, while  the  water  regains  a  small  part,  and 
after  some  days  will  again  yield  a  supply  of  active 
gas.  The  rate  of  recovery  and  decay  seem  to 
be  about  the  same  as  for  the  radium  emanation, 
and  this  suggests  that  the  active  material  is 
radium  in  minute  quantity. 

Again,  McLennan,  Rutherford  and  Cooke,  and 
Strutt  have  found  that  the  rate  of  leak  in  a 
closed  vessel  depends  on  the  nature  of  the  walls 
of  the  vessel.  Strutt  even  detected  differences 
with  different  samples  of  the  same  material. 
Cooke  diminished  the  rate  of  leak  in  a  brass 
electroscope  by  carefully  cleaning  the  walls.  Pro- 
bably this  result  is  to  be  explained  by  the  pre- 
sence of  slight  traces  of  some  active  emanation 
in  the  atmosphere,  and  the  consequent  excited 
activity  on  solid  materials.  The  excited  activity  is 
removed  by  cleaning,  but  the  fact  that  it  seems  to 
be  impossible  to  prevent  some  ionization  of  the 
air,  indicates  the  possibility  that  ordinary  materials 
are  faintly  radio-active. 

Evidence  pointing  to  a  similar  conclusion 
has  been  obtained  by  Thomson,  who  finds  that 
by  surrounding  an  electroscope  with  thick  layers 
of  different  substances,  the  leak  of  electricity 
may  be  increased  or  diminished  but  never  quite 
destroyed.  The  materials  themselves  seem  to 


218  PHYSICAL  SCIENCE 

emit  radiations  which  partly  compensate  the 
absorption  they  exercise  on  the  radiation  from 
surrounding  objects.  On  the  other  hand, 
boiling  many  finely  divided  substances  in  water 
failed  to  produce  any  trace  of  emanation. 
According  to  the  evidence  at  present  available, 
it  seems  probable  that,  while  all  bodies  are 
to  some  slight  extent  radio-active  and  emit 
ionizing  radiations,  emanations  are  emitted  only 
by  a  very  limited  number  of  elements. 

The  air  of  the  atmosphere  itself,  when  tested 
with  a  sensitive  electroscope,  is  found  to  possess 
a  slight  conductivity.  It  seems  likely  that 
this  effect  is  due  to  traces  of  some  radio-active 
substance,  whence  issue  the  radiations  which 
ionize  the  air.  The  rate  of  leak  of  electricity 
through  air  has  been  shown  by  Elster  and 
Geitel  to  be  greater  in  a  cave  or  cellar  than  in 
the  open ;  while  air  drawn  from  a  clay  soil 
contained  a  radio-active  emanation.  From  such 
experiments  we  know  that  traces  of  some  radio- 
active substance  are  present  in  many  places  in 
the  earth,  and  we  know  that  some  active  bodies 
emit  radiations  of  extremely  penetrating  nature. 
It  thus  seems  reasonable  to  believe  that  the 
slight  conductivity  which  appears  to  exist  at  all 
times  in  the  atmosphere  is  due  to  the  produc- 
tion of  gaseous  ions  by  the  action  of  stray 


RADIO-ACTIVITY  219 

radiations     proceeding     from     some     radio-active 
material,  near  or  far. 

Radium  seemed  at  one  time  destined  to  play 
a  useful  part  in  the  future  curative  treatment  of 
certain  diseases.  Rontgen  rays  have  occasionally 
been  employed  as  a  means  of  checking  the  spread 
of  cancer,  and  the  radiations  from  radium  also 
appeared  to  be  effective,  besides  being  applied  far 
more  easily  locally,  and  for  considerable  periods. 
Unfortunately  the  existence  of  such  benefits  re- 
mains doubtful. 

The  medicinal  springs  of  Bath  and  Buxton 
contain  radio-active  emanations,  while  radium 
itself  has  been  detected  in  the  solid  deposits  at 
Bath.  It  is  possible  that  the  curative  effects  of 
these  waters  is  caused  by  their  radio-activity,  and 
if  so,  the  uselessness  of  drinking  the  water,  when 
kept  and  removed  to  a  distance,  may  be  due,  more 
to  the  decay  of  the  activity  of  the  emanations,  than 
to  the  provident  imagination  of  the  local  authorities. 

Mr.  Soddy  has  suggested  that  inhaling  the 
emanations  of  radium,  or,  preferably,  of  thorium, 
might  prove  a  useful  way  of  treating  lung  disease. 
The  gentle  radio-activity  thus  obtained  is  perfectly 
under  control  by  varying  the  time  of  application  ; 
and  the  excited  activity  on  the  walls  of  the  lungs 
would  continue  the  treatment  in  a  milder  form  for 


220  PHYSICAL  SCIENCE 

some  hours  after  the  inhalation  had  ceased.  For 
surgical  purposes  also,  radium,  if  it  could  be  pre- 
pared fairly  pure  in  moderate  quantities,  would 
be  more  convenient  than  Rontgen  rays,  for  the 
production  of  which  complicated  and  expensive 
apparatus  is  needed,  apparatus,  too,  somewhat 
capricious  in  its  action. 

At  the  same  time  it  is  clear  that  the  use  of 
radium  is  attended  with  difficulty  and  danger. 
When  kept  near  the  skin  it  causes  sores,  of  a 
nature  not  yet  fully  understood,  which  only 
appear  some  days  afterwards,  while  if  caterpillars 
or  other  little  animals  are  confined  in  a  box  with 
a  small  quantity  of  a  radium  compound,  they 
die  in  a  few  hours. 

In  seeking  an  explanation  of  these  physiological 
effects,  recent  experiments,  due  to  Mr.  W.  B. 
Hardy,  must  be  noticed.  As  we  have  seen  in 
Chapter  IV.,  solutions  of  salts  and  acids,  which 
are  conductors  of  electricity,  possess  the  power  of 
coagulating  clear  solutions  of  colloidal  or  jelly-like 
substances  such  as  albumen  or  sulphide  of  arsenic, 
and  this  action  is  readily  explained  by  referring 
the  coagulative  action  to  the  electric  charges  on 
the  ions. 

The  influence  of  charged  ions  on  colloidal 
solutions  being  thus  made  clear,  Hardy  tried  the 
effect  of  exposing  a  very  sensitive  solution  of 


RADIO-ACTIVITY  221 

globulin;  a  substance  contained  in  the  living  tissue 
of  animals,  to  the  charged  particles  emitted  from 
radium,  which  so  readily  produce  ions  when 
passing  through  a  gas.  The  penetrating  ft  rays 
were  without  action,  but  the  easily  absorbed  a 
rays,  which  enter  a  film  of  the  liquid  when  it  is 
placed  near  a  radium  salt  with  no  screen  inter- 
posed, immediately  coagulated  the  globulin.  On 
the  other  hand,  the  ft  and  y  rays  were  found 
to  induce  certain  chemical  reactions,  liberating 
iodine  from  iodoform  in  presence  of  oxygen.  This 
change  is  also  produced  by  ordinary  light  and  by 
Rontgen  rays,  but  not  by  the  a  radiation.  These 
results,  physical  and  chemical,  may  explain  some 
of  the  curious  physiological  effects  of  radio-active 
substances. 

It  seems  unlikely  that  radium  will  ever  be  cheap 
enough  for  us  to  use  its  energy  as  a  source  of 
mechanical  power,  but  it  is  just  possible  that  the 
phosphorescence  of  sensitive  screens  in  the  neigh- 
bourhood of  a  radio-active  body  may  some  day 
act  as  a  practical  source  of  light.  In  this  way 
luminous  effects  would  be  obtained  directly  from 
a  store  of  energy  self-contained  and  practically 
inexhaustible,  whereas,  in  all  our  present  arrange- 
ments, light  is  derived  from  a  hot  body,  and  large 
quantities  of  energy  are  necessarily  wasted  in  main- 
taining the  incandescence. 


222  PHYSICAL  SCIENCE 

In  order  to  gain  some  insight  into  the  cause  of 
radio-activity,  we  must  now  examine  another  series 
of  phenomena  of  fundamental  importance,  which 
were  discovered  in  the  case  of  uranium,  by  Crookes 
and  by  Becquerel,  and  in  the  case  of  thorium  by 
Rutherford  and  Soddy.  By  definite  processes  of 
chemical  fractionisation,  somewhat  like  those  by 
which  radium  was  isolated  from  pitch-blende, 
products  can  be  obtained  in  minute  quantities 
from  uranium  and  thorium  many  times  more 
active  than  those  substances  themselves.  The 
uranium  and  thorium  from  which  those  products 
have  been  separated  lose  much  of  their  activity  ; 
the  radiation  they  then  emit  seems  to  be  an 
inseparable  property  of  the  elements  themselves, 
and  is  of  the  a  type  only.  To  the  separated  pro- 
ducts the  names  of  uranium-^  and  thorium-^ 
have  been  given.  The  important  point  is  this : 
if  these  X  products  be  kept  for  some  weeks  or 
months,  they  will  be  found  to  have  lost  their 
radio-active  properties,  while  the  original  samples 
of  uranium  or  thorium  will  have  become  as  active 
as  they  were  before  the  separation,  and  will  again 
emit  all  three  types  of  radiation.  The  rates  at 
which  the  processes  of  loss  and  gain  of  activity 
occur  have  been  carefully  studied  by  Rutherford 
and  Soddy,  and  shown  to  correspond  accurately 
with  each  other.  This  correspondence  is  clearly 


100 


RADIO-ACTIVITY  223 

shown  by  the  curves  in  Fig.  33,  which  give  the 
decay  of  activity  of  the  separated  uranium-Jf, 
and  the  recovery  of  the  residual  uranium.  Again 
we  see  that  the  total  amount  of  activity  remains 
constant,  and  is  not  affected  by  the  processes 
of  chemical  action. 

These  experiments  lead  to  a  definite  view  as  to 

URANIUM 


60 


2O  4O  6O  SO  1OO          12O  14O          16O 

Time     in,     days 

FIG.  33. 


the  source  of  the  radiations.  It  has  been  suggested 
that  the  energy  proceeding  from  radio-active  bodies 
is  obtained  by  drawing  on  some  unknown  radiation 
constantly  streaming  through  space,  a  radiation 
which  active  bodies  alone  have  the  power  of  ab- 
sorbing and  re-emitting  in  forms  capable  of  detec- 


224  PHYSICAL  SCIENCE 

tion  by  our  instruments.  But  the  activity  of  a 
radium  compound  is  found  to  be  normal  when 
the  compound  is  kept  within  a  thick  case  of  lead, 
which  would  probably  absorb  stray  radiation, 
and  moreover,  the  suggested  explanation  ignores 
the  results  of  experiments  such  as  those  just 
described,  which  show  that,  whenever  radio- 
activity exists,  the  active  material  is  always  slowly 
changing  into  some  other  substance,  which  has 
distinct  chemical  properties,  and  can  be  separated 
by  chemical  means  from  the  original  material. 
Thus,  in  the  case  of  thorium  compounds,  the 
radio-active  body  producing  most  of  the  effects 
usually  observed  is  not  really  thorium,  but  a  defi- 
nite substance  we  may  call  thorium-^,  which  is 
being  formed  at  a  constant  rate  from  the  bulk  of 
the  thorium,  and,  after  its  formation,  gradually 
loses  its  activity.  The  radio-activity  of  the  pure 
thorium  seems  to  be  a  consequence  of  its  change 
into  thorium-J^  and  to  accompany  that  change. 
The  activity  of  the  thorium-^,  in  a  similar  way, 
accompanies,  and  is  a  consequence  of,  its  continual 
change  into  other  bodies,  in  this  case,  the  thorium 
emanation.  The  constant  activity  of  a  thorium 
compound,  as  ordinarily  found,  is  thus  due  to  a 
balance  in  the  rate  of  production  of  the  active 
thorium-^T  and  the  rate  of  its  loss  of  radio- 
activity. 


RADIO-ACTIVITY  225 

What  view  are  we  to  take  of  the  changes  in  the 
thorium  or  uranium  which  result  in  the  formation 
of  the  X  products,  and  what  further  changes  must 
we  suppose  to  go  on  when  the  X  products  give 
rise  to  emanations  or  radiations  ?  Are  these 
changes  of  the  nature  of  ordinary  chemical  action, 
in  which  atomic  or  molecular  combinations,  or 
rearrangements  of  the  ato'ms  in  a  molecule,  are 
involved,  or  must  we  look  deeper  for  their 
causes  ? 

Three  essential  pieces  of  evidence  should  be 
considered  in  this  connection.  The  rate  at  which 
radio-active  power  is  gained  or  lost  depends  only 
on,  and  is  always  proportional  to,  the  total  amount 
of  active  material  at  any  instant  remaining  effec- 
tive ;  it  does  not  depend  on  the  concentration  of 
that  material.  For  instance,  if  the  activity  of  a 
quantity  of  thorium-^,  or  of  radium  emanation, 
be  examined,  it  will  be  found  to  decrease  during 
each  unit  of  time  by  the  same  fraction  of  the 
value  it  had  at  the  beginning  of  that  interval.  If, 
in  the  first  four  days,  the  activity  falls  to  half  its 
initial  value,  during  the  second  four  days  it  will 
fall  to  half  that  half-value,  or  to  one  quarter  of  the 
initial  value  ;  during  each  successive  four  days  the 
remaining  activity  is  halved,  the  process  being 
represented  by  a  curve  of  the  type  of  those  in 
Figs.  32,  33.  The  rate  of  decay  does  not  depend 


226  PHYSICAL  SCIENCE 

on  the  volume  which  the  material  occupies.  This 
mode  of  change  in  a  geometrical  progression, 
depending  only  on  the  total  amount  of  effective 
material  present  at  the  instant,  is  well  known  in 
chemical  processes.  In  such  processes  it  always 
indicates  that  the  reaction  is  an  alteration  going  on 
in  the  individual  molecules,  which  may  either  be 
dissociating  into  simpler  molecules,  or  be  suffering 
a  rearrangement  of  their  constituent  atoms.  Each 
molecule  undergoes  this  change  alone,  and  does 
not  react  with  other  molecules.  If,  on  the  other 
hand,  a  change  is  going  on,  in  which  combination 
or  rearrangement  between  two  reacting  systems  is 
involved,  whether  the  systems  consist  of  atoms  or 
molecules,  another  law  holds ;  and  the  rate  of 
change  is  found  to  increase  when  the  material  is 
concentrated  into  a  smaller  space,  so  that  the  two 
systems  are  more  closely  within  reach  of  each 
other.  In  the  phenomena  we  are  considering, 
then,  the  change  involves  one  system  only,  what- 
ever that  system  may  be. 

In  examining  the  further  question  thus  raised, 
we  are  at  once  confronted  with  the  remarkable 
fact  that  the  radio-activity  of  a  series  of  com- 
pounds of  any  radio-active  element  is  simply 
proportional  to  the  amount  of  the  element  which 
they  contain.  The  activity  of  the  element  is  not 
affected  by  its  state  of  combination,  or  by  very 


RADIO-ACTIVITY  227 

great  changes  in  the  physical  conditions,  such 
as  temperature,  which  play  a  large  part  in  de- 
termining ordinary  physical  or  chemical  equi- 
librium. As  we  have  seen,  this  remarkable  re- 
sult applies  not  only  to  the  emission  of  the 
"  rays/'  but  also  to  the  evolution  of  the  emana- 
tions which  proceed  from  some  of  the  radio- 
active elements  ;  the  differences  in  emanating 
power  have  been  traced  to  differences  in  the 
rate  at  which  the  emanations  can  escape  from 
the  various  compounds  under  various  conditions. 
The  law  of  decay  of  activity  shows  that  one 
reacting  system  only  is  involved;  these  further 
phenomena  show  that  the  system  does  not  alter 
with  the  changing  conditions  which  are  found 
to  affect  all  known  molecular  processes,  or  with 
the  state  of  combination  which  affects  the  physi- 
cal and  chemical  properties  that  control  the  be- 
haviour of  the  elements  in  all  other  respects. 
Moreover,  as  we  shall  see  later,  it  is  possible 
to  calculate  the  energy  liberated  by  a  given 
amount  of  radio-active  change.  This  energy  is 
at  least  five  hundred  thousand  times,  and  may 
be  ten  million  times,  greater  than  that  involved 
in  the  most  energetic  chemical  action  known. 

The  conclusion  is  thus  forced  on  us  that, 
in  radio-active  processes,  we  are  dealing  with 
changes  in,  the  atoms  themselves,  and  are  watch- 


228  PHYSICAL  SCIENCE 

ing  the  phenomena  which  accompany  a  true 
transmutation  of  the  elements.  The  continuity 
of  the  problems  which  present  themselves  to  the 
human  intellect  is  once  more  strikingly  demon- 
strated, for  surely  the  imagination  must  be  de- 
ficient which  does  not  see  in  these  transformations 
of  matter  a  partial  fulfilment  of  the  dreams  of 
the  mediaeval  alchemist. 

The  strength  of  any  hypothesis  lies  in  its 
power  of  adequately  co-ordinating  observed 
facts,  and  of  intelligently  forecasting  the  dis- 
coveries of  the  future.  If,  then,  we  accept  this 
new  revelation,  and  in  its  light  reconsider  the 
phenomena  we  have  already  discussed,  we  shall 
be  able  to  marshal  our  facts  in  orderly  array, 
while  the  few  privileged  pioneers  alone  can  tell 
how  much  assistance  they  have  already  received 
from  it  in  their  brilliant  achievements. 

Let  us  then,  in  terms  of  this  new  theory, 
re-state  the  results  which  we  have  already  de- 
scribed. All  radio-active  elements  have  very 
high  atomic  weights,  the  atom  of  radium,  for 
instance,  being  about  225  times  as  heavy  as 
that  of  hydrogen.  Radio-active  atoms  are  there- 
fore very  complex  structures,  and,  on  the  theory 
we  are  considering,  are  capable  of  breaking  down 
into  simpler  and  lighter  systems.  The  elements 
thorium  and  uranium  contain  some  few  atoms 


RADIO-ACTIVITY  229 

which,  at  any  moment,  are  disintegrating.  As 
we  have  seen,  the  activity  of  the  pure  separated 
thorium  or  uranium  consists  of  a  rays  only. 
Thus,  the  essential  process  of  the  radio-activity 
of  these  bodies  consists  in  the  emission  of  a 
rays,  the  disintegration  of  each  atom  resulting 
in  the  projection  of  one  or  more  a  particles  with 
a  velocity  about  one-twelfth  that  of  light,  while 
the  residues  break  down  into  new  and  simpler 
atoms,  which  are  themselves  in  a  state  of  insta- 
bility, and  are  known  to  us  as  thorium-^  and 
uranium-^f. 

The  further  transformation  of  these  bodies  is 
very  rapid,  their  activity  disappearing  in  a  time  to 
be  measured  in  days.  It  is  probable,  however,  that 
in  radium  we  possess  an  analogous  substance, 
also  an  intermediate  product  in  a  state  of  insta- 
bility, the  life  of  which  is  enormously  longer. 
The  primary  substance,  standing  to  radium  as 
thorium  stands  to  thonum-X,  is  at  present  un- 
certain ;  it  may  be  one  of  the  metals  which  are 
found  in  pitch-blende. 

As  in  the  formation  of  the  X  product,  the 
essential  process  in  the  radio-activity  which 
accompanies  its  disintegration  consists  in  the 
ejection  of  the  positively  charged  particle  which 
we  recognise  as  an  a  ray  by  its  power  of 
ionizing  a  gas  through  which  it  passes,  and  thus 


230  PHYSICAL  SCIENCE 

rendering  that  gas  a  conductor.  The  loss  of  this 
positive  particle  implies  a  change  in  the  atomic 
residue,  which  now,  in  the  case  of  uranium-^T, 
seems  to  lose  all  radio-active  properties,  and 
therefore  to  pass  out  of  reach  of  our  present 
powers  of  observation. 

In  compounds  of  radium  and  thorium,  how- 
ever, we  get  the  emanations  as  the  next  step  in 
the  process  of  atomic  dissociation.  These  bodies 
also  are  unstable,  that  is,  radio-active.  They 
emit  new  a  rays,  and  produce  the  excited 
activity  which  generally  appears  in  a  deposit  on 
the  walls  of  the  containing  vessels.  This  again 
breaks  down,  with  the  usual  accompaniment  of 
a  radiation.  The  decay  of  the  excited  activity 
on  a  rod,  exposed  for  a  very  short  time  to  the 
radium  emanation,  is  shown  in  Fig.  34.  The 
curve  is  a  complicated  one,  and  may  profitably 
be  compared  with  the  simple  curves  giving  the 
rate  of  decay  of  the  activity  of  uranium-A', 
the  curve  of  Fig.  33  on  page  223,  and  with 
the  curve  of  decay  of  the  radium  emana- 
tion, Fig.  32  on  page  215.  Rutherford  has 
shown,  however,  that  the  complex  curve  of  the 
decay  of  the  excited  activity  of  radium  can  be 
made  up  by  the  conjunction  of  three  constituent 
curves,  of  the  usual  typical  form  shown  by 
uranium  X.  This  indicates  no  less  than  three 


RADIO-ACTIVITY 


231 


successive  changes  in  the  radio-active  matter : 
(i),  a  rapid  initial  change,  accompanied  by 
radio-activity  ;  half  the  matter  changes  in  about 
three  minutes ;  (2),  a  slower  change  involving 


O  20         40          60  80          100 

Time,  in  Minutes 

FIG,  34. 


120 


no  radio-activity,  in  which  half  the  substance 
changes  in  thirty-six  minutes  ;  (3);  a  radio- 
active change,  half  completed  in  about  twenty- 
eight  minutes.  The  actual  curve  is  the  resultant 


232  PHYSICAL  SCIENCE 

of  these  three  processes,  which  are  going  on 
simultaneously.  It  is  remarkable  that  in  this 
way  we  can  detect  and  investigate  an  atomic 
change  involving  no  radio-activity.  We  cannot 
but  wonder  whether  some  such  disintegration  may 
not  be  taking  place  in  the  atoms  of  ordinary 
materials,  which  undergo  no  simultaneous  radio- 
active changes,  or,  at  all  events,  none  great 
enough  to  be  accurately  investigated  in  the  way 
by  which  alone  we  can  apprehend  at  present 
such  non-radio-active  changes. 

Therefore,  in  all,  five  certain  stages  have  been 
traced  in  the  disintegration  of  the  radium  atom, 
as  it  passes  successively  into  the  emanation  and 
the  three  types  of  matter  described  above. 

Evidence  of  a  still  further  change  is  also 
forthcoming.  Surfaces  exposed  to  the  radium 
emanation  retain  about  the  1/20,000  part  of  their 
initial  excited  activity,  apparently  for  an  indefi- 
nite period.  Rutherford  points  out  that  the  total 
amount  of  ionization  produced  by  the  whole 
process  of  disintegration  seems  to  be  about 
the  same  for  each  of  the  successive  changes 
of  radium  which  can  be  traced  throughout  its 
entire  course.  If  we  extend  this  conclusion  to 
the  persistent  excited  activity,  it  follows,  since 
the  activity  at  any  instant  is  about  the  1/20,000 
part  of  that  of  the  emanation,  that  the  life  of  the 


RADIO-ACTIVITY  233 

active  deposit  must  be  about  20,000  times  longer 
than  that  of  the  emanation,  which  decays  to  half 
its  intensity  in  four  days.  The  persistent  excited 
activity  would  thus  half  disappear  in  about  200 years. 
However  that  may  be,  we  are  now  nearing  the 
final  stage,  where  the  residue  left  by  the  successive 
changes  ceases  to  be  radio-active,  and  can  no 
longer  be  followed  by  our  instruments. 

The  quantities  of  matter  involved  in  any 
radio-active  change  are  excessively  minute,  and 
no  other  method  at  present  known  enables  us 
to  detect  the  final  inactive  products  as  they  are 
formed.  It  is,  however,  not  improbable  that,  by 
the  slow  accumulation  of  material  which  must 
of  necessity  go  on  when  a  radio-active  body  is 
kept  for  a  long  time,  the  inactive  products  will 
eventually  be  obtained  in  amounts  sufficient  to 
be  distinguished  by  the  spectroscope  or  even  by 
ordinary  chemical  analysis.  In  this  connection 
we  must  give  due  weight  to  the  fact  that  in  all 
radio-active  minerals  considerable  quantities  of 
the  gas  helium  are  occluded.  Sir  William  Ram- 
say and  Mr.  Soddy,  by  spectroscopic  methods, 
have  detected  the  presence  of  helium  in  the 
gases  evolved  from  a  sample  of  radium,  origin- 
ally prepared  from  pitch-blende  and  kept  as  a 
solid  for  some  months.  The  spectrum  of  helium 
was  invisible  when  the  emanation  was  first  collected 


234  PHYSICAL  SCIENCE 

and  examined,  but  it  soon  appeared  and  gradually 
increased  in  intensity  with  the  lapse  of  time. 

Similar  results  have  been  obtained  by  Dewar  and 
Curie,  who,  moreover,  appear  to  have  traced  the 
disappearance  of  a  minute  volume  of  the  emanation. 
This  has  been  explained  by  the  idea  that  the  -re- 
sulting helium,  being  projected  in  the  atomic  state 
with  great  velocity,  penetrated  the  glass  walls  of 
the  vessel  and  thus  occupied  no  volume.  The 
decrease  in  the  volume  of  a  minute  quantity  of 
emanation  has  also  been  observed  recently  by 
Ramsay  and  Soddy. 

Such  results  are  very  suggestive  :  it  seems 
difficult  to  avoid  the  conclusion  that  helium  is 
one  of  the  final  products  obtained  by  the  dis- 
integration of  the  radium  atom.  But  a  few 
years  since,  the  inactive  elements  of  the  group 
containing  argon  and  helium  were  revealed  to 
us.  It  now  seems  conceivable  that  we  are 
about  to  learn  from  them  a  secret  as  yet  un- 
disclosed by  other  elements,  the  secret  of  their 
origin.  The  density  of  helium,  about  double 
that  of  hydrogen,  suggested  the  possibility  that  the 
a  rays  themselves  consist  of  positively  electrified 
helium  atoms ;  but  further  evidence  is  needed  be- 
fore we  can  finally  solve  these  fascinating  problems 
of  the  ultimate  state  of  extinct  radio-active  matter. 

So  far  we  have  referred  to  the  a  type  of  radia- 


RADIO-ACTIVITY  235 

tion  alone,  and  this  type  seems  to  play  the  primary 
part  in  radio-active  change.  The  /5  and  y  radiations 
only  appear  when  the  series  of  atomic  transfor- 
mations accompanying  radio-activity  is  nearing  its 
completion.  With  uranium,  /3  and  y  rays  are 
emitted  only  by  the  substance  uranium  -X,  and 
this,  since  no  emanation  is  known,  must  represent 
the  last  radio-active  residue.  With  radium  and 
thorium,  the  excited  activity  due  to  the  emanations 
is  the  last  product  formed  before  the  radio-active 
properties  vanish  ;  and,  with  these  metals,  this 
excited  activity  freely  gives  rise  to  the  /5  and  y 
radiations,  which  are  absent  both  from  radium 
and  thorium  and  also  from  their  emanations.  The 
emission  of  B  and  y  rays  by  a  solid  radium  com- 
pound is  due  to  a  similar  excited  activity  produced 
in  the  solid  by  the  emanation  which  that  solid  is 
always  developing  and  absorbing.  This  explains 
the  gradual  increase  in  activity  of  a  radium  com- 
pound for  some  weeks  after  its  deposition  from 
solution.  The  increase  has  been  observed  both 
with  the  electroscope  and  by  measurement  of  the 
heat  evolved.  When  first  deposited,  we  get  the 
effect  of  the  solid  radium  salt,  which  gives  a  rays 
alone  ;  but,  as  the  emanation  is  gradually  pro- 
duced and  stored  in  the  solid,  not  only  does  the 
amount  of  a  radiation  grow  greater,  but  /3  and  y 
radiations,  due  to  the  excited  activity,  also  appear. 


236  PHYSICAL  SCIENCE 

The  nature  of  the  a  rays  still  awaits  final  elucida- 
tion. Their  positive  charges  seem  to  be  transient, 
and,  if  so,  their  calculated  masses  cannot  be  relied 
upon.  They  may  be  flights  of  helium  atoms ;  but 
their  penetrating  power  is  much  greater  than  that 
of  cathode  rays  with  equal  velocity,  and,  as  H.  A. 
Wilson  suggests,  they  may  consist  of  positive  elec- 
trons. The  large  apparent  mass  must  then  be 
due  to  the  size  of  the  radius,  minute,  even  compared 
with  that  of  the  negative  electron  (see  p.  283).  This 
and  other  points  remain  for  future  investigation  ; 
but  the  main  features  of  the  theory  we  have  out- 
lined, if  they  cannot  yet  be  said  to  be  as  firmly 
established  and  as  widely  accepted  as  some  older 
physical  generalisations,  have  at  all  events  pro- 
vided us  with  a  consistent  treatment  of  the 
subject,  and  constitute  an  effective  hypothesis  on 
the  lines  of  which,  it  is  safe  to  predict,  will  be 
continued  in  the  coming  years  the  researches 
which  have  already  so  notably  widened  our  out- 
look on  the  workings  of  nature. 

Such  is  the  theory  of  radio-activity  indicated  by 
the  remarkable  series  of  investigations  that  have 
followed  Becquerel's  original  discovery.  We  are 
led  to  refer  the  energy  liberated  to  transformations 
in  the  chemical  atoms,  and  to  recognise  clearly, 
what  has  long  been  suspected,  that  the  store  of 


RADIO-ACTIVITY  237 

energy  in  the  atoms  themselves  enormously  tran- 
scends the  energy  involved  in  ordinary  physical  or 
chemical  changes,  in  which  the  atoms  suffer  no 
alteration.  This  internal  atomic  energy,  then, 
must  be  looked  on  as  the  source  of  the  heat 
experimentally  detected  by  Curie  in  the  neigh- 
bourhood of  a  radium  compound.  Its  immediate 
cause  may  be,  partly  at  least,  the  internal  bom- 
bardment of  the  a  particles,  which,  shot  off  by  the 
radium  and  the  emanation  stored  in  it,  are  for 
the  most  part  absorbed  by  the  substance  itself. 
Rutherford  has  traced  the  increase  of  the  heat 
effect  in  radium  bromide  newly  precipitated  from 
solution,  and  has  shown  that  it  grows  pari  passu 
with  the  radio-activity  as  measured  electrically — a 
method  which,  as  we  have  seen,  depends  chiefly 
on  the  a  radiation. 

The  greater  part  of  the  radiation  coming  from 
a  solid  radium  compound  is  emitted  by  the 
stored  emanation  and  its  product,  the  excited 
activity.  The  emanation  can  be  extracted  only 
in  such  minute  quantities  that,  except  in  most 
exceptional  conditions,  its  radio-activity  alone 
reveals  to  us  its  existence.  As  we  have  seen, 
the  emanation  is  of  the  nature  of  a  dense  gas, 
half  of  any  quantity  of  which  would  be  trans- 
formed into  other  substances  in  about  four  days. 
Owing  to  this  process  of  change,  only  a  limited 


238  PHYSICAL  SCIENCE 

amount  of  emanation  could  be  obtained  from  a 
given  quantity  of  radium,  and  the  bubble  which 
could  be  evolved  from  the  small  supply  of  radium 
possessed  by  any  experimenter  would  be  too  minute 
to  be  visible,  except  by  the  most  refined  and  sen- 
sitive methods  of  investigation.  Could  a  cubic 
inch  of  the  radium  emanation  be  obtained,  the 
radiation  from  it  would  be  so  powerful  that  the 
vessel  used  to  contain  the  gas  would,  in  all  proba- 
bility, be  fused  instantly. 

By  the  methods  we  have  already  described,  it  is 
possible  to  determine  the  mass  and  the  velocity  of 
the  projected  particles,  and  therefore  to  calculate 
their  kinetic  energy.  From  the  principles  of  the 
molecular  theory,  we  know  that  the  number  of 
atoms  in  a  gramme  of  a  solid  material  is  about 
io20.  Four  or  five  successive  stages  in  the  dis- 
integration of  radium  have  been  recognised  ;  and, 
on  the  assumption  that  each  of  these  involves  the 
emission  of  only  one  particle,  the  total  energy  of 
radiation  which  one  gramme  of  radium  could 
furnish  if  entirely  disintegrated  seems  to  be  enough 
to  raise  the  temperature  of  io8  grammes,  or  about 
100  tons  of  water,  through  one  degree  C. 
This  is  an  under-estimate  ;  it  is  possible  that  it 
should  be  increased  ten  or  a  hundred  times.  As  a 
mean  value,  we  may  say  that,  in  mechanical  units, 
the  energy  available  for  radiation  in  one  ounce  of 


RADIO-ACTIVITY  239 

radium  is  sufficient  to  raise  a  weight  of  something 
like  ten  thousand  tons  one  mile  high. 

An  interesting  problem  then  arises  for  solution. 
From  Curie's  determination  of  the  heat  evolved,  or 
from  estimates  of  the  number  of  ions  produced  by 
each  a  particle  as  it  flies  through  air  till  its  energy 
is  absorbed,  we  may  calculate  approximately  the 
energy  radiated  in  a  given  time.  Hence  we 
deduce  the  possibility  of  estimating  the  rate  at 
which  the  observed  radiations  of  a  piece  of  radium 
would  decrease — that  is,  the  length  of  life  of  radium 
as  its  atoms,  one  after  another,  undergo  disinte- 
gration. On  the  minimum  estimate  we  have  given, 
Rutherford  calculates  the  life  of  radium  as  a  few 
thousand  years  ;  and,  although  it  is  probable  that 
this  is  less  than  the  truth,  it  seems  that  a  gramme 
of  the  material  would  so  diminish  in  the  course  of 
a  few  hundred  thousand  years,  that  the  activity  of 
the  residue  would  cease  to  be  a  measurable  quan- 
tity. The  activity  of  uranium  and  thorium  is  so 
much  smaller  than  that  of  radium  that  their  lives 
must  be  perhaps  a  million  times  longer,  and  the 
amount  of  matter  disintegrated  in  a  year  a  million 
times  less.  We  are  thus  again  impressed  with  the 
exceeding  delicacy  of  our  measurements  of  radio- 
activity, by  which  we  can  detect  in  a  few  seconds 
the  results  of  changes  to  be  demonstrated  by  the 
balance  only  after  the  lapse  of  many  thousand  years. 


240  PHYSICAL  SCIENCE 

Forasmuch  as  the  life  of  radium  is  less  than 
the  probable  age  of  the  minerals  in  which  it 
is  found,  we  may  surmise  that  the  element  is 
continually  formed  by  the  disintegration  of  other 
atoms.  If  this  be  so,  the  radio-activity  of  uranium 
minerals  will  reach  a  constant  value  when  the 
rate  of  formation  of  radium  is  equal  to  its  rate 
of  decay.  Radium  is  found  only  in  association 
with  ores  of  uranium,  and  in  amount  roughly 
proportional  to  the  quantity  of  uranium  present. 
A  suspicion  thus  arises  about  the  parentage  of 
radium.  As  far  as  is  known,  uranium  under- 
goes only  one  change  in  which  a  rays  are 
emitted,  while  four  such  changes  have  been 
detected  in  radium.  The  rates  of  formation 
and  decay  being  the  same,  it  would  seem  that, 
when  radio-active  equilibrium  is  reached,  the 
activity  of  the  mineral  should  be  about  four  times 
that  of  uranium.  It  is  remarkable  that,  allowing 
for  traces  of  other  active  elements  such  as 
thorium,  this  is  about  the  actual  value  of  the 
activity  of  the  best  pitch-blende. 

It  will  now  be  clear  that,  on  the  theory  which 
has  been  put  forward,  we  are,  while  investigating 
a  radio-active  body,  in  reality  watching  the  process 
of  the  evolution  of  matter.  Radio-active  sub- 
stances, themselves  unstable,  may  have  been 
formed  by  the  disintegration  of  parent  atoms, 


RADIO-ACTIVITY  241 

which  are  unknown  to  us,  and,  indeed,  may  now 
be  non-existent  on  our  globe.  Radio-activity 
denoting  an  unstable  state,  it  is  probable  that 
the  total  amount  of  it  in  the  world  is  constantly 
diminishing,  as  the  atoms  of  the  active  elements 
gradually  pass  into  inactive  forms.  Perhaps  in 
former  ages  nearly  all  matter  was  intensely  radio- 
active ;  and  mankind  has  discovered  these  pheno- 
mena only  in  the  last  cosmical  moments  of  a  few 
thousand  or  million  years  before  they  cease  for 
ever  to  manifest  their  existence  in  the  striking 
manner  which  has  made  radium  so  remark- 
able. 

When  we  trace  in  this  way  the  creation  and 
evolution  of  new  elements,  it  is  impossible  to  resist 
wondering  whether  the  process  of  change,  so  far 
observed  to  an  appreciable  extent  only  in  a  few 
radio-active  bodies,  may  not  in  reality  be  a  general 
property  of  matter,  though  in  other  cases  possessed 
in  such  an  infinitesimal  degree  that  it  almost  tran- 
scends the  delicate  means  of  detection  that  are 
now  at  our  disposal.  As  we  have  seen,  experi- 
mental evidence  is  not  altogether  wanting  in 
favour  of  such  a  supposition.  We  must,  at  any 
rate,  cease  to  regard  matter  as  essentially  eternal 
and  unalterable  ;  the  possibility  of  its  undergoing 
a  continual  though  slow  process  of  evolution  is 
glearly  before  us. 

Q 


242  PHYSICAL  SCIENCE 

A  striking  property  of  radio-active  change  is  our 
inability  to  produce  it,  or  even  to  modify  its  course, 
by  any  of  the  powerful  means  within  the  resources 
of  modern  physical  science.  The  highest  tem- 
perature we  can  employ,  the  intense  cold  of  liquid 
air,  are  equally  useless.  The  observation  that  the 
activity  of  radium  is  independent  of  the  concen- 
tration of  the  material  shows  that  the  disintegration 
of  one  part  of  the  substance  is  not  accelerated  by 
the  radiation  from  another  part.  Even  under  the 
fierce  and  continuous  bombardment  of  the  atomic 
projectiles  hurled  forth  by  radium,  and  the  sharp 
musketry  of  its  corpuscular  /3  rays,  the  residual 
atoms  are  unaffected.  They  remain  unchanged 
by  the  action  of  any  external  agency,  till,  in  the 
fulness  of  time,  their  own  internal  processes  result 
in  instability,  and  from  the  shattered  fragments  of 
each  radium  atom,  as,  in  its  turn,  it  breaks 
asunder,  new  elements  emerge. 

By  investigating  radio-active  changes,  we  can 
trace  the  transmutation  of  the  elements  ;  we  can 
watch  the  evolution  of  matter ;  but  we  have  not 
yet  found  the  philosopher's  stone  which  brings 
these  processes  under  our  control.  It  would  be 
rash  to  predict  that  our  impotence  will  last  for 
ever,  but  at  present  there  are  no  signs  of  its 
removal.  Still,  strange  things  happen  nowadays, 
and  yet  stranger  things  may  be  seen  by  future 


RADIO-ACTIVITY  243 

generations.  It  is  conceivable  that  some  means 
may  one  day  be  found  for  inducing  radio-active 
change  in  elements  which  are  not  normally  subject 
to  it.  Professor  Rutherford  has  playfully  suggested 
to  the  writer  the  disquieting  idea  that,  could  a 
proper  detonator  be  discovered,  an  explosive  wave 
of  atomic  disintegration  might  be  started  through 
all  matter  which  would  transmute  the  whole  mass 
of  the  globe,  and  leave  but  a  wrack  of  helium 
behind.  Such  a  speculation  is,  of  course,  only  a 
nightmare  dream  of  the  scientific  imagination, 
but  it  serves  to  show  the  illimitable  avenues  of 
thought  opened  up  by  the  study  of  radio- 
activity. 


CHAPTER    VII 

ATOMS    AND    .ETHER 

"  Oh,  dear  !  what  can  the  matter  be  ?  " 

— Old  Song. 

OUR  primary  conception  of  matter  as  continuous 
in  time  and  space  fails  to  correspond  with  pheno- 
mena which  are  perceived  as  soon  as  inquiry  passes 
beyond  the  most  elementary  stages.  The  expan- 
sion of  a  quantity  of  gas  without  assignable  limit 
can  hardly  be  represented  mentally  if  the  gas  is 
thought  of  as  a  homogeneous  substance  com- 
pletely rilling  the  space  in  which  it  exists.  We 
cannot  imagine  that  the  same  amount  of  substance 
fills  equally  at  different  times  volumes  different 
from  each  other.  The  immediate  difficulty  dis- 
appears if  we  suppose  the  gas  to  consist  of  a 
number  of  discrete  particles,  which  can  be 
pressed  nearer  together  or  allowed  to  move 
farther  apart. 

The  phenomena  of  diffusion,  too,  clearly  indicate 
that  liquids  and  gases  must  consist  of  particles  in 
motion  relatively  to  each  other,  capable  of  pene- 


ATOMS  AND  ^THER  245 

trating  the  interspaces  between  the  similar  par- 
ticles of  contiguous  bodies.  A  vessel  filled  with 
hydrogen  and  a  vessel  filled  with  oxygen,  when 
opened  into  each  other,  soon  contain  an  equal 
mixture  of  the  two  gases,  while  two  solutions  in 
contact  gradually  become  of  uniform  concentration 
throughout.  Nor  are  such  processes  confined  to 
fluids.  Sir  William  Roberts-Austen  has  shown 
that  gold,  if  placed  in  intimate  contact  with  lead, 
will  diffuse  at  ordinary  temperatures  to  such  an 
extent  that,  after  the  lapse  of  some  years,  it  can 
be  detected  in  the  lead  by  chemical  analysis  at 
distances  of  a  millimetre  or  more  from  the 
surface  of  contact.  Chemical  analysis  is  by  no 
means  one  of  the  most  sensitive  methods  of  re- 
search, and  to  be  discovered  in  this  way  the  gold 
must  have  migrated  to  a  considerable  extent. 

Again,  all  our  present  conceptions  of  the  nature 
of  heat  rest  on  the  view  that  it  is  a  form  of  energy 

—the  energy  of  agitation  of  particles  too  small  to 
be  recognised  or  controlled  individually  by  ordinary 
mechanical  means.  No  hypothesis  previously  pro- 
posed offers  any  approach  to  a  satisfactory  ex- 
planation. 

While  the  most  obvious  phenomena  thus  point 
consistently  to  the  conception  of  the  grained 
structure  of  matter,  the  more  recondite  branches  of 

physical  science  indicate  the  same  conclusion  by 


246  PHYSICAL  SCIENCE 

evidence  which,  in  its  cumulative  effect,  is  irresis- 
tible. The  phenomena  of  liquid  electrolysis,  no 
less  than  those  of  gaseous  discharge  and  radio- 
activity, have  been  successfully  co-ordinated  and 
explained  by  ionic  hypotheses  which  are  an  extreme 
form  of  molecular  theory.  Indeed,  in  the  experi- 
ments devised  by  C.  T.  R.  Wilson,  the  clouds 
deposited  on  ionic  nuclei  seem,  by  no  very  violent 
stretch  of  the  imagination,  to  bring  the  individual 
ions  themselves  within  reach  of  our  imperfect 
senses,  and  Crookes  has  shown  that  the  molecular 
bombardment  of  the  individual  a  particles  from 
radium  may  possibly  be  rendered  visible  by  the 
scintillations  produced  on  a  fluorescent  screen 
of  zinc  sulphide. 

Turning  to  chemistry,  we  are  again  impelled  to 
molecular  conceptions  by  the  familiar  evidence  on 
which  rests  Dalton's  atomic  theory.  It  is  true 
that  here  the  possibility  of  an  alternative  explana- 
tion, based  on  the  principles  of  energetics  alone, 
is  now  before  us.  Mixtures  possessing  a  maximum 
or  minimum  melting  or  boiling-point  change  their 
state  without  change  in  composition  of  either 
phase.  The  particular  composition  at  which  this 
mode  of  change  occurs  depends  in  general  on  the 
physical  conditions,  such  as  pressure.  If,  however, 
as  a  limiting  case,  variation  in  conditions  is  with- 
out effect,  the  system  would  be  classed  as  a  com- 


ATOMS  AND  AETHER  247 

pound  or  an  element — a  compound  if  the  constancy 
extends  only  over  a  limited  range,  an  element  if 
no  known  variation  of  conditions  will  alter  the 
composition.  Whether  or  not  the  need  for 
atomic  conceptions  may  thus  be  banished  from 
chemistry,  such  conceptions  must  still  remain  as 
an  alternative  explanation. 

Truth  may  possess  many  aspects,  and  provision- 
ally we  may  accept  safely  the  idea  of  the  molecular 
structure  of  matter.  It  remains  to  consider  whether 
it  is  possible  to  obtain  any  exact  knowledge  of  the 
dimensions  of  this  structure,  that  is,  of  the  number 
of  molecules  we  must  suppose  to  exist  in  a  cubic 
centimetre,  and  of  the  size  of  the  molecular  indi- 
viduals. 

It  is  clear  that  the  molecules  must  be  at  least  as 
small  as  the  most  minute  piece  of  matter  we  can 
prepare  and  recognise,  and  in  many  ways  it  is 
possible  to  obtain  substances  in  a  very  fine  state 
of  division.  Gold  leaf  can  be  beaten  out  till  its 
thickness  does  not  exceed  the  millionth  part  of 
an  inch,  while  the  deep  blue  colour  of  thin  smoke 
coming  from  a  wood  fire  shows  that  the  particles 
therein  are  able  to  distinguish  selectively  the 
various  waves  making  up  a  beam  of  white  light, 
and  must  therefore  be  comparable  in  minuteness 
with  the  lengths  of  those  waves. 


248  PHYSICAL  SCIENCE 

Such  results  as  these,  while  fixing  an  upper 
limit  to  the  size  of  molecules,  are  powerless  to 
assist  in  the  determination  of  a  lower  limit,  smaller 
than  which  the  inter-molecular  distances  cannot  be. 
Such  inferior  limits  can,  however,  be  determined, 
and  to  one  of  the  methods  by  which  they  have 
been  obtained  —  one  due  to  Lord  Kelvin  —  we 
will  now  turn. 

A  soap  bubble  always  tends  to  contract  and 
diminish  its  area,  and  therefore,  in  order  to 
increase  its  size,  it  is  necessary  to  do  work  against 
the  force  of  contraction  to  an  amount  which  may 
be  calculated  by  measuring  the  surface  tension  of 
the  film.  Adding  the  energy  required  to  prevent 
the  film  from  cooling  during  its  extension,  we  can 
calculate  the  total  work  absorbed  per  unit  increase 
of  area.  By  continual  extension  it  would  be  pos- 
sible to  expend  an  unlimited  quantity  of  work,  as 
long  as  no  change  in  the  nature  of  the  film  took 
place  under  the  influence  of  the  progressive  expan- 
sion and  consequent  attenuation  of  the  film.  The 
point  at  which  it  is  natural  to  expect  that  some 
change  would  occur  is  that  moment  when  the  two 
sides  of  the  film  have  been  brought  so  near  to 
each  other,  by  the  process  of  continual  thinning, 
that  the  outside  faces  confining  the  film  come 
within  range  of  each  other's  molecular  forces. 
But,  however  far  the  film  be  extended,  it  is  evident 


ATOMS  AND  AETHER  249 

that,  as  long  as  it  remains  a  film,  less  work  must 
be  used  than  could  otherwise  be  expended  in  eva- 
porating the  film  and  converting  its  substance  into 
steam,  since  by  this  means  its  molecules  would 
be  separated  completely  from  each  other's  sphere 
of  influence.  The  value  of  this  latter  amount 
of  work  is  known  from  other  experiments,  and  is 
the  latent  heat  of  evaporation  of  the  substance  of 
the  film,  which  is  composed  almost  entirely  of 
water.  It  is  possible,  therefore,  to  calculate  for 
the  film  a  hypothetical  thickness,  certainly  less 
than  the  critical  thickness  at  which  it  would  begin 
to  show  new  properties  owing  to  the  approach  of 
the  opposite  faces  within  molecular  distances. 
Numerical  results  show  that  this  limiting  thickness 
may  be  put  at  about  io~8  of  a  centimetre.  There 
are  thus  not  more  than  ten  million  molecules 
in  a  row  in  a  length  of  a  millimetre,  and  two 
hundred  million  in  the  space  of  an  inch.  The 
numbers  in  the  corresponding  volumes  will  be 
found  by  cubing  these  values  ;  a  cubic  centimetre 
of  water  contains  not  more  than  io24  molecules. 
This,  as  we  have  indicated,  is  a  maximum  estimate  ; 
it  is  possible  that  the  number  is  less. 

As  already  suggested,  the  interdiffusion  of  gases 
also  leads  to  a  molecular  conception  of  their 
structure,  and  from  the  observed  values  of  the 
coefficients  of  diffusion,  and  of  the  allied  property, 


250  PHYSICAL  SCIENCE 

viscosity,  it  is  possible,  from  the  principles  of  the 
kinetic  theory,  to  calculate  more  exactly  the  num- 
ber of  molecules  in  a  cubic  centimetre  of  a  gas. 
The  results  of  the  investigation  indicate  about 
2.5  x  io19  molecules  per  cubic  centimetre.  Since 
water,  the  liquid,  is  about  1200  times  denser  than 
its  vapour,  it  follows  that  a  cubic  centimetre  of 
water  contains  about  3  x  io22  molecules,  a  num- 
ber which  may  profitably  be  compared  with  the 
maximum  estimate  given  above.  Such  figures  do 
indeed  convey  little  to  the  mind  ;  but  it  may  be 
useful  to  remember  that  the  thinnest  line  clearly 
visible  in  a  good  microscope — a  line  with  a  thick- 
ness approaching  the  hundred-thousandth  of  a 
centimetre  —  would  need  about  three  hundred 
molecules  to  stretch  across  it  from  side  to  side. 
Thus  the  molecular  structure  of  matter  is  not 
immeasurably  finer  than  magnitudes  which,  with 
the  aid  of  modern  instruments,  our  senses  are 
enabled  to  apprehend. 

Our  mental  picture  of  matter,  then,  is  that  of 
a  discontinuous  substance  ;  we  can,  moreover, 
form  some  notion  of  the  number  of  grains  in 
a  given  volume,  and  we  know  some  of  the 
chemical  properties  of  the  individual  grains.  But 
what  is  the  nature  of  these  particles  ?  Are  they 
similar  in  kind  to  the  matter-in-bulk  they  com- 


ATOMS  AND  ^THER  251 

pose,  or  do  the  properties  of  matter-in-bulk 
appear  as  a  consequence  of  the  collaboration 
of  vast  numbers  of  particles  essentially  different 
in  nature  from  any  lump  of  matter  we  can 
touch  or  see  ?  Again,  are  the  particles  which 
make  up  different  kinds  of  matter  different  from 
each  other,  or  has  all  matter  a  common  con- 
stituent ?  Are  the  different  elements  composed  of 
identical  particles  of  which  the  number  and 
arrangement  form  the  determining  factors  of  the 
chemical  atoms  ? 

Such  questions  have  puzzled  mankind  from 
early  times,  and,  until  theories  began  to  be 
founded  on  facts  and  tested  by  experiment,  the 
track  of  history  is  strewn  with  the  speculative 
hypotheses  of  the  metaphysicians  and  the  poets. 
Here  and  there  a  lucky  guess  or  shrewd  sug- 
gestion chances  to  agree  with  the  views  which 
represent,  temporarily  it  may  be,  the  conclusions 
of  experimental  science.  It  is  curious  and  in- 
teresting that,  to  many  highly  educated  people, 
the  problems  connected  with  the  constitution  of 
matter  are  better  known  by  such  triumphant 
proofs  of  the  sagacity  and  scientific  insight  of 
some  Greek  philosopher  than  from  the  more  de- 
finite conceptions  tentatively  put  forward  by  a 
Kelvin,  a  Larmor,  or  a  ].  ].  Thomson,  from  the 
basis  of  experimental  knowledge. 


252  PHYSICAL  SCIENCE 

The  problems  at  issue  could  not  even  be 
formulated  profitably  till  the  work  of  Dalton 
and  Avogadro  had  fixed  our  ideas  of  atoms  and 
molecules.  In  the  light  of  present  knowledge, 
we  define  an  atom  to  be  the  smallest  particle 
of  matter  which  can  take  part  in  chemical 
action,  or  enter  into  the  chemical  structure  of 
a  compound.  It  is  the  ultimate  chemical  unit ; 
anything  smaller  than  an  atom,  if  such  a  par- 
ticle is  to  be  found,  would  have  no  chemical 
properties,  and  it  would  play  no  distinguishable 
part  in  ordinary  chemical  action.  The  atom  is, 
moreover,  defined  as  the  unit  of  the  chemical 
elements.  An  atom  of  a  compound  is  a  meaning- 
less term ;  the  atoms  of  water,  for  instance, 
would  be,  not  water,  but  hydrogen  and  oxygen. 

As  to  the  outward  chemical  nature  of  atoms, 
qua  atoms,  physics  has  nothing  to  say ;  but 
molecules,  on  the  other  hand,  may  be  re- 
garded either  from  a  chemical  or  from  a  physi- 
cal aspect.  Chemically  they  are  the  ultimate 
units  of  the  compound,  the  smallest  parts  of  that 
compound  which  can  exist  and  still  retain  the 
properties  of  the  compound.  Any  farther  sub- 
division would  result  in  the  liberation  of  the 
elements.  Physically,  on  the  other  hand,  mole- 
cules are  the  smallest  particles  of  matter  which 
act  as  wholes  in  the  incessant  irregular  move- 


ATOMS  AND  AETHER  253 

ments  which  the  particles  of  matter  are  always 
undergoing.  The  energy  of  these  molecular 
movements  is  the  energy  of  heat ;  and  in  the 
most  striking  case,  that  of  a  gas,  the  impact  of 
the  molecules  on  the  walls  of  the  containing 
vessel  gives  the  physical  explanation  of  the  pres- 
sure which  the  gas  exerts.  It  is  evident  that 
the  physical  molecule  may  contain  one  or  more 
chemical  atoms.  Clear  evidence  shows  that  in 
well-known  gases  such  as  oxygen  and  hydrogen, 
the  molecule  consists  of  two  atoms,  while  the 
vapours  of  some  metals,  mercury,  for  example, 
possess  monatomic  molecules. 

Thus  the  relations  between  atoms  and  mole- 
cules are  ascertained,  and  further  inquiry  must 
deal  with  the  intimate  structure  of  the  atom, 
as  the  more  fundamental  unit. 

The  essence  of  Dalton's  great  discovery  was 
that  the  relative  chemical  combining  weights  of 
the  different  elements  lead  directly  to  a  know- 
ledge of  the  relative  weights  of  the  atoms  of 
those  elements.  Since  Dalton's  time  it  has  been 
recognised  that  the  atoms,  in  the  chemical  sense 
of  the  word,  of  different  elements  must  have 
different  weights  and  different  properties.  If, 
then,  we  look  for  some  common  constituent 
composing  the  different  elementary  substances 
known  to-  chemistry,  we  must  look  within  the 


254  PHYSICAL  SCIENCE 

atom;  we  must  cease  to  regard  it  as  the  ultimate 
unit,  and  examine  the  internal  structure  of  the 
atom  itself ;  we  must  abandon  the  etymological 
meaning  of  the  word,  retaining  it  only  for  its 
historic  associations. 

On  arranging  the  elements  in  order  of  their 
atomic  weights,  Mendeleef  discovered  that  periodic 
relations  become  apparent  between  the  physical 
and  chemical  properties,  elements  with  similar 
properties  recurring  at  constant  intervals.  This 
periodicity  was  so  marked  a  feature  that  it  was 
possible  to  arrange  the  elements  in  groups, 
in  which  the  various  properties  were  possessed 
by  the  individual  members  to  a  greater  or  less 
extent,  according  to  their  position  in  the  groups. 
It  was  even  possible  successfully  to  predict 
the  atomic  weight,  properties,  and  compounds  of 
undiscovered  elements  from  knowledge  of  the 
behaviour  of  their  neighbours,  which  were  situated 
round  empty  spaces  in  the  periodic  table. 

The  periodic  law  suggests  a  common  origin 
for  the  elements,  and  indicates  that,  as  we  pass 
from  light  to  heavy  atoms,  we  are  going  from 
simple  to  complex  structures  containing  different 
numbers  of  some  common  sub-atom.  The  atomic 
weights  of  many  elements  are  nearly  simple  mul- 
tiples of  that  of  hydrogen,  and  Prout  supposed 
that  hydrogen  was  the  ultimate  basis  from  which 


ATOMS  AND  .ETHER  255 

other  elements  took  their  rise.  Accurate  experi- 
ments have  not  eliminated  the  small  divergence 
of  several  atomic  weights  from  whole  numbers, 
and  Prout's  hypothesis  in  its  original  form  has 
long  been  discarded  ;  but  the  idea  of  some 
common  constituent  in  the  different  elements  has 
much  evidence  in  its  favour,  and  only  waited  for 
definite  experimental  confirmation  to  be  received 
as  the  natural  conclusion  of  many  promising 
speculations. 

For  the  first  time,  in  1897,  such  definite 
experimental  evidence  was  given  by  Professor 
J.  J.  Thomson,  who,  in  the  remarkable  series 
of  researches  described  on  pages  172  to  180, 
clearly  showed  that,  in  the  cathode  rays  of  a 
vacuum  tube,  we  can  detect  corpuscles  possess- 
ing about  one-thousandth  part  of  the  mass  of 
the  lightest  atom  known  to  chemistry,  that  of 
hydrogen.  These  corpuscles  were  shown  to  be 
identical,  whatever  the  nature  of  the  residual  gas 
in  the  tube,  and  whatever  the  metal  employed 
as  electrode.  The  corpuscles  are  common  to 
all  kinds  of  matter,  and  the  mind  at  once  sees 
in  them  the  long-sought  ultimate  basis  from 
which  all  atoms  are  made. 

To  explain  the  phenomena  of  radiation,  and 
all  the  complex  systems  of  lines  which  appear 
in  the  spectra  of  the  elements,  it  is  necessary 


256  PHYSICAL  SCIENCE 

to  imagine  these  corpuscles,  not  locked  together 
in  a  close-packed  conglomerate,  but  moving  in 
oscillatory  or  orbital  motion  under  the  influence 
of  their  mutual  forces.  The  emission  of  cor- 
puscles from  the  radio  -  active  elements  also 
indicates  such  movement,  for  it  is  unlikely  that 
the  high  velocities  with  which  the  corpuscles 
are  hurled  away  should  have  been  impressed  on 
them  at  the  instant  of  ejection. 

Thomson  has  published  recently  (March  1904) 
a  mathematical  investigation  of  the  conditions 
of  stability  of  systems  of  revolving  corpuscles, 
and  has  thereby  deduced  in  a  most  remarkable 
manner  many  of  the  properties  of  the  different 
chemical  atoms.  He  supposes  any  one  atom  to 
consist  of  a  uniform  sphere  of  positive  electri- 
fication, the  structure  of  which  is  not  specified, 
and  of  a  number  of  negatively  charged  cor- 
puscles revolving  in  orbits  within  that  positive 
sphere,  under  the  influence  of  the  attraction  of 
the  positive  electricity  and  of  their  own  mutual 
repulsions. 

A  similar  problem  was  long  ago  attacked  by 
Mayer  by  means  of  experiment.  A  number  of 
little  magnetised  needles  were  thrust  through 
corks,  and  were  allowed  to  float  on  the  surface 
of  water  with  their  axes  vertical.  The  similar 
poles  of  all  the  magnets  were  directed  upwards. 


ATOMS  AND  ^THER  257 

and  thus  the  resultant  force  between  the  mag- 
nets was  a  repulsion.  High  above  the  water 
was  placed  a  powerful  bar  magnet,  with  that 
pole  downwards  of  which  the  magnetisation  was 
opposite  in  kind  to  that  of  the  upward  poles 
of  the  little  floating  magnets.  This  large  pole 
attracted  inwards  all  the  little  poles  pointing 
upward,  and  thus  the  magnets  were  drawn 
towards  the  centre  by  the  attraction  of  the  big 
magnet  suspended  above  them,  and  at  the  same 
time  were  repelled  from  the  centre  by  their 
mutual  repulsions.  Under  the  influence  of  these 
two  forces  they  assumed  positions  of  equili- 
brium. 

Mayer  found  that  as  long  as  the  number  of 
little  magnets  was  not  more  than  five,  they 
arranged  themselves  in  a  single  ring,  but  that, 
on  increasing  the  number  to  six,  a  discontinuity 
of  arrangement  was  observed,  the  single-ring 
structure  ceased  to  be  stable,  and  the  magnets 
placed  themselves  with  five  in  a  ring  and  one  at 
the  centre.  This  two-ring  configuration  persisted 
as  more  magnets  were  added,  till  the  number  rose 
to  fourteen,  with  five  in  the  middle  ring  and  nine 
in  the  outer  circle.  With  fifteen  magnets  this 
arrangement  in  its  turn  became  unstable,  and  a 
three-ring  system  appeared. 

Thomson  has  now  overcome  the  difficulties  of 

R 


258  PHYSICAL  SCIENCE 

the  mathematical  analysis,  and  has  shown  that 
similar  phenomena  of  disposition  must  appear  in 
the  system  which,  as  described  above,  he  imagines 
may  correspond  with  the  atom.  Here  also,  discon- 
tinuities in  arrangement  will  appear,  and,  when 
certain  definite  numbers  of  corpuscles  have  come 
together,  an  additional  ring  will  be  formed. 
Periodic  likenesses  in  structure  also  arise  ;  thus, 
for  example,  the  system  of  sixty  corpuscles  has  its 
internal  parts  arranged  similarly  to  the  system  of 
forty,  in  fact,  it  has  the  same  arrangement  as  the 
system  of  forty  with  an  additional  ring  of  twenty 
corpuscles  placed  round  it.  In  the  same  way,  the 
system  of  forty  corpuscles  corresponds  with  the 
system  of  twenty-four,  with  an  external  ring  of 
sixteen,  while  twenty-four  arrange  themselves  in 
the  manner  of  eleven  with  an  additional  ring. 
Such  similarities  of  arrangement  will  give  to  the 
system  in  which  they  occur  similarities  of  periods 
of  vibration,  and  explain  the  homologous  series  of 
lines  which  are  found  in  the  spectra  of  elements 
lying  in  the  same  group  of  Mendeleeff's  periodic 
table. 

In  the  table  which  follows  all  the  arrangements 
of  corpuscles  are  given  with  twenty  in  the  outer 
ring.  Fifty-nine  is  the  smallest  number  of  cor- 
puscles which  place  themselves  with  an  outer 
ring  of  twenty,  while,  with  numbers  greater  than 


ATOMS  AND  AETHER 


259 


sixty-seven,  the  outer  ring  will  contain  more  than 
twenty  corpuscles. 


'• 

II. 

HI. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

Total  number  of 
corpuscles. 

59 

60 

61 

62 

63 

64 

65 

66 

67 

2 

3 

3 

3 

3 

4 

4 

5 

5 

Number  of  corpuscles 
in  successive  rings. 

8 

13 
16 

13 
16 

9 

13 
16 

13 
17 

10 

13 
17 

10 

13 
17 

10 

14 

17 

10 
H 
17 

10 

15 
17 

20 

20 

20 

20 

20 

20 

20 

20 

20 

1 

The  stability  of  the  outer  ring  is  small  when  the 
number  of  corpuscles  within  it  is  small,  and,  in 
the  example  given,  if  the  number  in  the  inner 
rings  falls  below  thirty-nine,  the  outer  ring  will 
collapse,  and  a  new  arrangement  appear,  with  an 
outer  ring  of  nineteen  instead  of  twenty.  With 
fifty-nine  corpuscles,  although  the  outer  ring  is 
unstable,  no  corpuscle  can  be  detached  per- 
manently, for  a  break  up  of  the  arrangement 
would  follow. 

Passing,  however,  to  the  next  member  of  the 
table,  the  atom-model  containing  sixty  corpuscles, 
the  stability  of  the  outer  ring,  although  increased, 
is  still  small,  and  a  corpuscle  is  easily  detached. 
This  process  would  carry  away  a  negative  unit  of 
electricity,  and  leave  the  atom  positively  electrified. 
Only  one  corpuscle  can  be  lost,  for  the  subtraction 


260  PHYSICAL  SCIENCE 

of  more  than  one  would  reduce  the  total  below 
fifty-nine,  and  the  general  arrangement  would 
change. 

In  Chapter  IV.  it  was  shown  that  the  pheno- 
mena of  the  electrolysis  of  liquids  indicates  a  close 
connection  between  the  electric  charges  of  atoms 
and  their  chemical  valency,  and  it  seems  probable 
that  chemical  combination  is  an  effect  of  the 
inter-atomic  electric  forces.  Thus  the  system  of 
sixty  corpuscles,  as  described  above,  will  act  as  the 
model  of  a  monovalent,  strongly  electro-positive 
atom. 

From  the  system  of  sixty-one  corpuscles,  two 
may  be  detached ;  though,  the  stability  of  the  outer 
ring  being  greater,  with  less  facility.  Here  then 
we  have  a  less  electro-positive,  divalent  atom. 

Similarly  the  arrangement  of  sixty-two  cor- 
puscles corresponds  with  a  trivalent  atom,  with 
still  less  marked  electro-positive  properties. 

At  the  other  end  of  the  series,  the  stability  of 
the  outer  ring  is  very  great,  and,  with  the  system 
containing  sixty-six  corpuscles,  for  example,  it  will 
be  possible  for  a  negative  corpuscle  to  attach  itself 
to  the  outside  of  the  system  with  no  change  in 
general  arrangement.  The  system,  then,  acts 
as  an  atom,  monovalent,  and  strongly  electro- 
positive. But  a  corpuscle  added  to  the  collection 
of  sixty-seven  will  produce  a  change  in  the  outer- 


ATOMS  AND  .ETHER  261 

most  ring,  which  must  now  contain  twenty-one 
corpuscles  and  will  be  very  unstable.  The  sixty- 
seven  group  then  has  no  valency,  in  this  resem- 
bling the  elements  at  the  other  end  of  the  row. 

To  the  group  of  sixty-five  corpuscles  it  is 
possible  to  add  two  beyond  the  normal  number 
without  general  rearrangement :  it  represents  a 
divalent  electro-negative  element.  The  system  of 
sixty-four  corpuscles  corresponds  with  a  trivalent 
electro-negative  atom,  and  so  on. 

Now  let  us  compare  these  theoretical  results 
with  the  first  two  rows  of  chemical  elements  in 
MendeleefF s  periodic  table,  as  set  forth  below : — 

[i.  ii.  in.  iv.  v. 

Helium.     Lithium.      Beryllium.          Boron.        Carbon. 
Neon.       Sodium.      Magnesium.    Aluminium.    Silicon. 

VI.  VII.  VIII.  IX. 

Nitrogen.         Oxygen.          Fluorine.         Neon. 
Phosphorus.       Sulphur.         Chlorine.        Argon. 

The  first  and  the  last  element  in  each  series 
have  no  chemical  valency,  while  the  valency  of  the 
others  rises  in  order,  so  that  we  get  monovalent, 
divalent,  trivalent,  and  tetravalent  atoms,  as  we 
pass  from  the  ends  of  the  rows.  The  elements 
at  the  left  end  are  electro-positive,  and  those 
towards  the  right  electro-negative. 

The  concordance  of  Thomson's  theoretical 
scheme  with  the  periodic  properties  of  the 


262  PHYSICAL  SCIENCE 

chemical  elements  themselves  —  a  concordance 
almost  Satanic  in  its  exactness  and  verisimilitude 
— forces  us  irresistibly  to  believe  that,  in  these 
hypothetical  systems  of  revolving  corpuscles  we 
have  models  which  reflect  in  some  really  intimate 
way  the  structure  of  the  mysterious  originals. 

An  outstanding  difficulty  of  interpretation  re- 
mains in  the  positive  electrification  required  to 
keep  the  negative  corpuscles  in  their  orbits ;  and 
this  difficulty  waits  for  future  elucidation. 

At  first  sight  we  may  well  say  that  Thomson's 
corpuscle — one  of  the  latest  conceptions  of  science 
— does  but  carry  us  back  to  the  ideas  and  specu- 
lations of  Democritus,  and  justify  the  glorification 
of  those  ideas  in  the  poem  of  Lucretius,  though 
internal  evidence  seems  to  show  that  Lucretius 
himself  did  not  find  the  explanation  entirely  con- 
vincing : — 

"  Nee  me  animi  fallit  Graiorum  obscura  reperta 
Difficile  inlustrare  Latinis  versibus  esse." 

If,  however,  in  one  aspect  these  modern 
corpuscles  may  resemble  the  hard,  impenetrable 
atoms  of  the  Greek  philosopher  and  the  Latin  poet, 
such  a  resemblance  vanishes  when  we  identify 
them  with  the  disembodied  charges  of  electricity, 
mathematically  studied  by  Larmor  and  Lorentz. 
If  the  corpuscle  is  a  negative  electron — a  disem- 


ATOMS  AND  AETHER  263 

bodied  ghost — an  electric  charge — we  enter  a 
region  of  knowledge  the  bare  existence  of  which 
was  unknown  to  the  ancients. 

The  hard  particle  of  Democritus,  which,  as  late 
as  the  age  of  Newton,  still  served  as  a  working 
hypothesis,  gradually  failed  to  respond  to  the 
demands  made  on  its  constitution  by  both  philo- 
sophers and  physicists,  in  their  search  for  a  con- 
ceptual model  of  the  chemical  atom.  Pictures  of 
mere  lumps  of  stuff,  similar  in  kind  to  the  per- 
ception of  matter-in-bulk  given  by  our  senses, 
were  no  help  to  the  theories  of  the  metaphysician, 
while  the  complexity  of  structure  demanded  by 
the  facts  of  radiation  as  disclosed  by  the  spectro- 
scope, showed  that  an  atom  must  be  capable  of 
many  and  various  modes  of  vibration. 

In  extreme  opposition  to  the  hard  impenetrable 
sphere  of  Democritus,  we  have  Boscovich's  ideal- 
istic conception  of  atoms  as  centres  of  force. 
This  theory  gave  too  little  scope  for  definite 
development  to  serve  permanently  as  a  useful 
working  hypothesis,  and,  in  face  of  the  phenomena 
of  atomic  radiation,  it  too  seemed  insufficient. 

It  seemed  that  a  real  advance  had  been  made 
when  Lord  Kelvin  applied  the  theory  of  vortex 
rings,  developed  by  Von  Helmholtz  and  himself, 
to  explain  the  properties  of  the  atoms  of  matter. 
A  smoke  ring,  blown  in  air,  soon  dies  away,  but 


264  PHYSICAL  SCIENCE 

even  this  evanescent  thing,  while  it  lasts,  shows  a 
definite  separation  from  the  surrounding  medium, 
and  maintains  an  independent  existence.  Air  is 
an  imperfect  fluid,  and  movement  in  it  is  resisted 
by  the  frictional  forces  due  to  its  viscosity,  but,  if 
we  imagine  the  air  replaced  by  a  hypothetical 
perfect  fluid,  in  which  there  is  no  viscosity,  vortex 
rings,  once  formed,  will  persist  for  ever.  In  a 
fluid  not  quite  perfect,  their  life  will  be  long, 
though  not  eternal. 

Here  then  was  a  striking  representation  of  some 
of  the  most  important  properties  of  the  chemical 
atoms.  The  structure  of  interlacing  systems  of 
vortex  rings  gave  sufficient  complexity  to  explain 
radiation,  the  infinite  possibilities  of  variation  in 
number  and  arrangement  of  the  rings  would 
account  for  the  relations  between  different  atoms 
as  manifested  in  the  periodic  law,  while  the  per- 
sistence of  matter  could  be  explained  if  a  perfect 
or  nearly  perfect  fluid  existed  as  the  basis  of  the 
vortex  motion. 

At  this  point  we  reach  for  the  first  time  in 
our  inquiry  the  idea  of  an  all-pervading  medium 
— an  idea  which  has  played  such  a  large  part 
in  the  development  of  physical  science,  that 
a  considerable  digression  will  be  necessary. 
Newton  explained  the  phenomena  of  light  by  a 


ATOMS  AND  AETHER  265 

corpuscular  theory.  He  supposed  that  streams  of 
corpuscles  were  projected  from  luminous  objects, 
and  produced  the  sensation  of  sight  by  imping- 
ing on  the  nerves  of  the  eye.  Newton's  theory 
was  ultimately  abandoned,  mainly  for  two  reasons. 
The  phenomena  of  refraction  could  only  be  ex- 
plained by  it  on  the  assumption  that  the  cor- 
puscles travelled  more  quickly  in  dense  media 
than  in  air,  and  this,  always  improbable,  was 
eventually  disproved.  On  the  other  hand,  the 
theory  failed  to  explain  the  phenomena  of  inter- 
ference and  diffraction  of  light,  except  by  the 
addition  of  so  many  arbitrary  supplementary  hypo- 
theses, that,  in  the  end,  it  was  borne  down  by 
the  weight  of  its  own  superstructure. 

This  illustrates  a  case,  oft  recurring,  not  only  in 
the  realm  of  science,  where  men  have  been  de- 
ceived and  led  to  form  opinions  wide  of  the  truth 
through  the  agency  of  certain  resemblances  to  that 
truth.  The  corpuscular  theory  of  light  was  put 
aside,  but  not  before  it  had  appreciably  retarded 
the  progress  of  science.  The  master-mind,  the 
originator  of  the  theory,  had  been  withdrawn 
before  altered  circumstances  and  increased  know- 
ledge reversed  the  weight  of  evidence.  He  who 
would  have  been  the  first  to  detect  the  want  of 
harmony,  the  first  to  move  on  to  new  conceptions 
in  the  search  for  truth,  by  the  irony  of  fate,  be- 


266  PHYSICAL  SCIENCE 

came  for  a  time,  in  virtue  of  his  intellectual  supre- 
macy, a  stumbling-block  to  his  weaker  brethren,  and 
an  impediment  to  the  cause  he  had  most  at  heart. 

In  recent  years  the  discovery  of  radio-activity 
has  revealed  to  us  particles  very  like  those  that 
Newton  used  to  explain  ordinary  light.  The  /3 
rays  from  radium  are  projected  particles  moving 
with  velocities  nearly  approaching  that  of  light 
itself.  Newton's  inscrutable  insight,  amounting 
almost  to  an  instinctive  knowledge  of  Nature,  has 
again  been  demonstrated.  His  corpuscles  cannot, 
indeed,  explain  the  phenomena  of  ordinary  light  ; 
but  similar  corpuscles  we  find  do  exist,  and  their 
properties  as  set  forth  by  Newton  are  not  so  un- 
like those  actually  occurring  in  the  working  of 
Nature  as  men  have  assumed  throughout  the  years 
which  separate  the  establishment  of  the  undulatory 
theory  of  light  from  the  discovery  of  radio-activity. 

The  corpuscular  theory  of  light  was  replaced  by 
a  theory  of  waves  in  a  medium  which  already  had 
been  recognised  by  Newton  as  a  necessary  addition 
to  his  idea  of  corpuscles.  Newton's  difficulty, 
which  caused  him  to  reject  the  undulatory 
hypothesis,  namely,  the  rectilinear  propagation  of 
light,  and  the  consequent  possibility  of  sharp 
shadows,  was  finally  overcome  by  Fresnel  and 
Young,  who  showed  that  shadows  were  the 
result  of  the  minuteness  of  the  wave-lengths  of 


ATOMS  AND  .ETHER  267 

light  as  compared  with  the  dimensions  of  ordinary 
obstacles.  This  cleared  the  way  for  the  wave 
theory  as  already  formulated  by  Huygens,  and 
there  arose  a  definite  physical  need  for  the  exact 
specification  of  an  aether  or  luminiferous  medium, 
pervading  all  space,  and  the  interstices,  if  not  the 
substance,  of  material  objects.  Such  a  medium, 
indeed,  had  long  been  imagined  by  philosophers, 
as  a  means  of  transmitting  actions  from  one  body 
to  another,  but  its  use  as  a  physical  explanation  of 
the  phenomena  of  light  first  indicated  some  of  its 
necessary  properties.  The  reflection  of  light  from 
the  surface  of  a  glass  plate,  or  its  passage  through 
certain  doubly  refracting  crystals,  such  as  tour- 
maline, modifies  the  light,  which  acquires  pro- 
perties not  the  same  on  all  sides  of  the  emergent 
beam.  No  wave  system  in  which  the  direction  of 
vibration  is  in  the  direction  of  propagation  can 
show  such  differences,  for  the  waves  must  be  alike 
on  all  sides  of  their  path,  and  it  follows  that  the 
luminous  vibrations  must  be  transverse  to  the 
direction  in  which  the  rays  are  travelling.  Trans- 
verse waves  imply  a  certain  amount  of  rigidity  or 
elasticity  of  shape  in  the  medium — such  elasticity 
as  is  possessed  by  solids  alone.  No  fluid  when 
distorted  has  any  tendency  to  return  to  its  original 
form  ;  it  cannot  transmit  waves  which  depend  on 
mere  distortional  displacements.  Waves  in  a  fluid 


268  PHYSICAL  SCIENCE 

must  be  waves  of  compression  and  expansion,  in 
which  the  direction  of  vibration  is  in  the  direction 
of  propagation. 

If,  then,  it  is  to  carry  a  transverse  wave-motion 
of  an  ordinary  mechanical  kind,  the  luminiferous 
aether  must  possess  some  of  the  properties  of  a  solid, 
and  one  of  the  great  problems  of  aethereal  physics 
consists  in  formulating  a  medium  possessing  the 
necessary  rigidity.  Any  elastic  jelly  theory  leads 
to  obvious  difficulties  when  the  passage  of  matter 
through  the  aether  is  considered,  a  passage  which 
often  proceeds  with  high  velocity,  but,  as  far  as 
observation  goes,  is  entirely  unimpeded.  Rays  of 
light  from  the  stars  appear  to  reach  the  earth  in 
straight  lines,  suffering  no  deflection  on  passing 
through  the  aether  outside  the  atmosphere  near  the 
earth.  This  result  suggests  that  the  luminiferous 
medium  is  not  disturbed  by  the  movement  through 
it  of  the  earth  with  a  velocity  of  eighteen  miles  a 
second — the  speed  with  which  the  earth  moves 
round  the  sun.  On  the  other  hand,  the  passage 
of  light  over  the  surface  of  the  earth  is  not  affected 
by  a  change  in  direction  relative  to  the  earth's 
total  motion,  the  velocity  of  the  light  is  the  same 
whether  it  is  passing  with  or  against  the  motion 
of  the  earth.  This  result  indicates  at  first  sight  a 
conclusion  opposed  to  that  formerly  reached, 
namely,  that  the  aether  is  at  rest  relatively  to  the 


ATOMS  AND  ^THER  269 

surface  of  the  earth  and  is  dragged  along  with  the 
ground  as  it  moves.  It  is  possible  to  reconcile 
these  results  by  certain  suppositions  as  to  the 
effect  of  moving  matter  on  the  absolute  velocity 
of  light  within  it,  but  the  general  dynamical 
problem  of  constructing  a  model  of  the  aether  on 
ordinary  mechanical  ideas  of  wave  propagation  has 
never  been  satisfactorily  accomplished. 

As  long  as  the  aether  was  invoked  only  to 
explain  the  phenomena  of  light,  the  difficulties  of 
interpretation  might  well  suggest  doubts  about  the 
fundamental  hypothesis  as  to  its  existence,  but  when 
Clerk  Maxwell  showed  that  it  was  possible  to  ex- 
plain the  phenomena  of  the  electro-magnetic  field 
by  an  aether  having  properties  identical  with  those 
of  the  luminiferous  medium,  the  evidence  for  both 
theories  was  strengthened  almost  indefinitely.  Max- 
well proved  mathematically  that  the  velocity  of  an 
electro-magnetic  wave  through  free  space  deter- 
mined the  relative  magnitudes  of  certain  electric 
units,  so  that  by  comparing  the  values  of  the  units 
the  velocity  could  be  calculated.  Experiment 
showed  that  the  velocity  was  the  same  as  that  of 
light ;  light  became  an  electro-magnetic  pheno- 
menon, and  optical  science  a  branch  of  electricity. 
Many  years  afterwards,  Maxwell's  great  work  was 
confirmed  by  the  direct  experiments  of  Hertz,  who 
detected  the  existence,  and  measured  the  speed, 


270 


PHYSICAL  SCIENCE 


of  electro-magnetic  waves,  thus  laying  the  founda- 
tions on  which  the  practical  art  of  wireless  tele- 
graphy is  based. 

If  we  accept  the  view  that  an  atom  is  composed 
of  a  large  number  of  corpuscles  in  orbital  or 
oscillatory  motion,  the  electro-magnetic  radiation 

which      constitutes 
P  light  must  take  its 

rise  from  the  acce- 
lerations of  these 
corpuscles  as  they 
revolve  in  their 
orbits. 

Faraday's  con- 
ception of  tubes-  of 
electric  force,  which 
— ^-  we  have  used  on 
page  169  to  eluci- 
date the  theory  of 
Rontgen  rays,  may  here  be  revived  in  order  to 
explain  the  radiation  of  ordinary  light.  As  long 
as  the  charged  corpuscle  is  moving  forward  with 
uniform  velocity,  it  carries  its  attendant  tubes 
with  it  in  a  steady  manner,  and  no  radiation 
occurs.  When  it  is  suddenly  stopped,  as  we  have 
seen,  an  electro-magnetic  pulse  spreads  out  from 
it,  travelling  with  the  velocity  of  light.  Within 


o     o' 


FIG.  35. 


ATOMS  AND  .ETHER  271 

the  sphere  covered  by  this  pulse,  the  tubes  of 
force  are  rectified,  so  as  to  correspond  with 
the  new  position  of  the  corpuscle  at  rest,  while 
outside  it,  in  regions  as  yet  unaffected  by  the 
change  in  velocity,  the  tubes  are  still  moving  for- 
ward with  the  original  speed  of  the  corpuscle. 
In  the  pulse  itself,  then,  the  electric  tubes  are  bent 
more  or  less  at  right  angles  to  the  direction  of 
propagation  of  the  pulse,  which  spreads  out  from 
the  corpuscle  as  centre.  When  tubes  move,  a 
magnetic  force  is  produced  at  right  angles  both 
to  their  length  and  to  their  direction  of  motion ; 
and  thus,  in  the  thickness  of  the  pulse,  a  magnetic 
force  exists,  also  at  right  angles  to  the  direction  of 
propagation  of  the  pulse,  that  is,  in  the  plane  of 
the  advancing  wave-front,  and,  in  that  plane,  at 
right  angles  to  the  direction  of  the  electric  force. 
The  pulse  is  thus  an  electro-magnetic  disturbance, 
and,  as  we  have  seen,  is  probably  the  physical 
interpretation  of  a  Rontgen  ray. 

Now,  if,  instead  of  imagining  the  moving  cor- 
puscle suddenly  brought  to  rest,  we  suppose  that 
it  is  reversed  in  its  path,  and  that  this  reversal 
occurs  periodically,  so  that  the  corpuscle  performs 
simple  harmonic  vibrations,  we  get,  instead  of  a 
single  thin  pulse,  a  series  of  less  abrupt  but  re- 
gularly recurring  alternations  propagated  out  from 
the  corpuscle  as  centre.  Each  Faraday  tube  is 


272  PHYSICAL  SCIENCE 

set  into  oscillation  at  its  inner  end,  and  transverse 
waves  travel  outwards  along  it,  just  as  waves 
travel  along  a  stretched  cord,  when  one  end  is 
oscillated  periodically  by  the  hand.  The  dis- 
tribution of  electric  and  magnetic  force  in 
the  advancing  wave-front  is  exactly  the  same 
as  in  the  case  of  the  sudden  pulse  already 
studied :  we  get,  in  fact,  a  series  of  regular 
aethereal  waves,  in  which  there  are  electric  and 
magnetic  forces,  both  in  the  plane  of  the  wave- 
front,  and  at  right  angles  to  each  other  in  that 
plane.  But  such  an  arrangement  is  precisely  that 
required  to  explain  the  phenomena  of  light. 

In  the  simple  case  we  have  taken,  the  corpuscle 
oscillates  backwards  and  forwards  in  a  straight 
path  :  the  vibrations  travel  as  tremors  along  the 
tubes  of  force  in  one  plane  only  ;  the  resultant 
light  is  plane  polarized.  In  the  more  general 
case,  we  must  suppose  that  the  corpuscle  oscillates 
in  a  circular,  or  elliptical  orbit,  and  the  tubes  of 
force  will  be  displaced  in  corresponding  motions  ; 
the  tremors  running  along  them  will  no  longer  be 
simple  to  and  fro  movements,  but  points  on  the 
tubes  will  describe  curved  paths.  These  paths 
continually  change  as  the  orbit  of  the  corpuscle 
changes,  and  we  get  a  complete  model  of  the 
propagation  of  common,  non-polarized  light. 

Faraday  tubes,  it  is  clear,  give  a  very  powerful 


ATOMS  AND  .ETHER  273 

and  convenient  method  of  studying  the  phenomena 
of  the  electro-magnetic  field,  and  indications  are  not 
wanting  that  they  represent  something  more  than 
a  useful  mathematical  fiction.  If  the  structure  of 
the  electric  field  is  discontinuous  in  reality,  as  our 
tube-picture  of  it  indicates  ;  if  the  electric  and 
magnetic  effects  of  a  charge  of  electricity  are  in 
reality  exerted  throughout  the  surrounding  space 
by  means  of  discrete  tubes  of  force — vortex  fila- 
ments in  the  aether,  or  whatever  they  may  actually 
be — an  advancing  wave  of  light  must  be  discon- 
tinuous also.  If  we  could  look  at  such  a  wave 
from  the  front,  and  magnify  it  millions  of  millions 
of  times,  we  should  see,  not  a  uniform  field  of  illu- 
mination, but  a  number  of  bright  specks  scattered 
over  a  dark  ground.  Each  tube  of  force  would 
convey  its  own  tremors,  and  these  would  consti- 
tute light,  but  between  them  would  lie  undisturbed 
seas  of  aether. 

Such  an  idea  about  the  nature  of  a  wave-front 
of  light  is  very  unexpected  and  surprising.  We 
are  inclined  at  once  to  relegate  our  tubes  of  force 
to  a  museum  of  conceptual  curiosities.  But  it  is 
a  remarkable  thing  that  certain  evidence  in  favour 
of  the  discontinuous  nature  of  a  wave-front  of 
light  really  does  exist.  It  is  impossible  to  examine 
the  luminous  effects  with  enough  magnification  to 
investigate  the  question,  but,  as  we  have  seen, 

3 


274  PHYSICAL  SCIENCE 

ultra-violet  light,  and  still  more  effectively  Rontgen 
rays,  are  capable  of  ionizing  a  gas  through  which 
they  pass.  Here,  it  is  the  molecules  of  the  gas 
which  are  affected,  and,  in  examining  the  ionizing 
power  of  the  rays,  we  are  in  effect  using  on  them 
a  microscope  of  molecular  dimensions. 

If  the  wave-front  of  a  Rontgen  pulse  were  con- 
tinuous, all  the  molecules  of  the  gas  would  be 
subject  to  the  same  disturbance.  But,  even  with 
the  strongest  ionizing  agency,  nothing  like  one 
molecule  in  a  million  is  found  to  be  affected. 
Thus,  if  the  wave-front  be  continuous,  we  must 
suppose  that  it  is  only  those  very  few  molecules 
which  are  in  some  peculiarly  receptive  state  that 
are  ionized.  The  stability  of  a  molecule  is  greatly 
affected  by  temperature,  and  if  a  critical  limit  of 
stability  were  needed  for  a  molecule  to  become 
ionized  by  the  rays,  we  should  expect  that  the 
ionizing  power  would  rapidly  increase  with  the 
temperature.  Recently  Mr.  McClung  has  shown, 
however,  that  temperature  has  no  appreciable 
effect.  This  curious  result  indicates  that  the 
ionizing  action  is  independent  of  the  state  of 
stability  of  the  molecule,  and  prevents  us  from 
finding  in  this  way  an  explanation  of  the  small 
number  of  the  ionized  molecules  in  the  path  of 
the  rays. 

It   is  possible  that  some  other  rare  condition, 


ATOMS  AND  .ETHER  275 

unaffected  by  temperature,  may  be  the  necessary 
preliminary  to  ionization  by  incident  radiation  ; 
but  it  is  also  possible  that  the  explanation  of  the 
smallness  of  the  ionization  is  to  be  sought  in  the 
idea  that  the  advancing  wave  is  discontinuous, 
and  is  composed  of  a  number  of  parallel  tremors 
running  along  discrete  tubes  of  force.  The  tubes 
of  force  being  scattered  at  wide  intervals  through 
the  aether,  comparatively  few  molecules  would  lie 
in  their  paths,  and  only  a  few  would  be  affected 
by  waves  running  along  the  tubes.  Matter  has 
been  analysed  into  discrete  particles  ;  electricity 
has  been  shown  to  be  made  up  of  indivisible 
units  ;  and  now  it  seems  probable  that  light  in 
physical  reality,  as  well  as  in  text-books  of  optics, 
is  composed  of  a  number  of  separate  rays. 

From  the  time  of  Maxwell  onwards  electro- 
magnetic considerations  have  formed  an  essential 
part  of  any  theory  of  the  aether.  It  is  certain 
that  luminous  and  electro-magnetic  radiations 
are  essentially  the  same  in  kind,  and  only  differ 
in  the  length  of  the  waves.  These  waves  are 
very  much  longer  when  produced  by  ordinary 
electro-magnetic  apparatus  than  when  waves  of 
light  are  emitted  by  the  electro-magnetic  pro- 
cesses of  atomic  vibration.  We  may,  of  course, 
cease  to  try  to  represent  the  properties  of  the 


276  PHYSICAL  SCIENCE 

aether  by  means  of  any  imaginary  mechanical 
model,  and,  regarding  light  as  a  system  of  elec- 
tro-magnetic waves,  push  the  inquiry  no  further. 
Such  a  mode  of  formulation  might  be  satisfac- 
tory if  we  restricted  ourselves  to  the  phenomena 
already  mentioned,  but  at  least  two  considera- 
tions prevent  our  resting  content  with  a  mere 
series  of  electro-magnetic  equations  as  a  final 
explanation.  While  radiation-effects  of  all  kinds 
may  be  successfully  co-ordinated,  no  conception 
is  thus  given  of  the  nature  of  a  static  electric 
charge,  or  of  an  ordinary  electric  current,  and 
there  seems,  on  this  mode  of  representation,  no 
means  of  attacking  the  problem  of  the  nature 
of  gravitation,  which  must  some  day  be  explained 
in  terms  of  the  universal  medium,  if  that  medium 
is  to  survive  as  a  permanent  conception  in  physi- 
cal science. 

Attempts  are,  therefore,  still  being  made  to 
describe  ideal  models  which  shall  represent  the 
properties  of  the  aether  by  familiar  mechanical 
conceptions.  If  such  a  model  is  successfully 
constructed,  it  will  not  necessarily  represent  the 
actual  structure  of  the  aether  ;  that  is  not  its 
object.  The  primary  function  of  such  a  suc- 
cessful model  is  to  justify  our  theory  of  the 
aether  as  expressed  in  Maxwell's  electro-magnetic 
equations,  in  the  other  equations  requisite  to 


ATOMS  AND  AETHER  277 

explain  electric  charges  and  currents,  and,  if 
possible,  to  suggest  an  explanation  of  gravitation 
also. 

Nowadays,  the  tendency  is  to  give  up  the 
old  elastic  solid  view  of  the  aether,  and  to 
secure  the  necessary  rigidity  in  another  way. 
A  top  when  spinning  possesses  rigidity  of  posi- 
tion. It  maintains  its  vertical  position  against 
the  effects  of  its  weight,  and  any  displacement 
from  the  vertical  is  followed  by  definite  oscilla- 
tions around  the  mean  position.  These  pheno- 
mena can  best  be  studied  in  the  gyroscope, 
which  has  now  found  a  practical  application  in 
the  Whitehead  torpedo,  where  a  direct  course 
is  kept  by  the  tendency  of  a  spinning  wheel  to 
maintain  its  axis  of  rotation  undeviated.  On 
these  principles,  Lord  Kelvin  has  described  a 
gyrostatic  aether,  in  which  the  rigidity  is  secured 
by  the  motion  of  some  still  more  primitive 
material. 

Following  the  line  of  thought  indicated  by  Lord 
Kelvin  with  his  conception  of  the  vortex  atom,  we 
now  conceive  matter  to  be  an  aethereal  manifesta- 
tion. But  the  simple  vortex  ring  itself  has  failed 
to  meet  the  demands  made  upon  it.  "  The  fluid 
vortex  atom,"  says  Larmor,  "  faithfully  represents 
in  many  ways  the  permanence  and  mobility  of 


278  PHYSICAL  SCIENCE 

the  sub-atoms  of  matter ;  but  it  entirely  fails 
to  include  an  electric  charge  as  part  of  their 
constitution.  According  to  any  aether  theory, 
static  electric  attraction  must  be  conveyed  by 
elastic  action  across  the  aether,  and  an  electric 
field  must  be  a  field  of  strain,  which  implies 
elastic  quality  in  the  aether  instead  of  complete 
fluidity :  the  sub-atom  with  its  attendant  electric 
charge  must  therefore  be  in  whole  or  in  part  a 
nucleus  of  intrinsic  strain  in  the  aether,  a  place 
at  which  the  continuity  of  the  medium  has  been 
broken  and  cemented  together  again  (to  use  a 
crude  but  effective  image)  without  accurately 
fitting  the  parts,  so  that  there  is  a  residual 
strain  all  round  the  place." 

It  will  be  noted  that  any  such  theory,  by 
which  matter,  the  subject  of  experimental  me- 
chanics, is  explained  as  an  aethereal  manifestation, 
changes  the  point  of  view  from  which  we 
regard  mechanical  models  of  the  aether  itself. 
-^Ether,  being  now  regarded  as  a  sub-material 
medium,  is  not  necessarily  described  by  the  ex- 
perimental laws  with  which  the  facts  of  ordinary 
mechanics  conform.  In  dealing  with  the  aether, 
we  are  on  an  entirely  different  plane,  and  have 
no  right  to  assume  that  a  mechanical  model  of 
its  properties  is  possible.  However,  as  our 


\ 

ATOMS  AND  AETHER  279 

dynamical  science  is  based  on  the  phenomena 
of  matter,  we  continue  to  describe  the  aether  in 
terms  of  semi-mechanical  models,  though,  strictly 
speaking,  the  mere  statement  in  mechanical  terms 
of  the  problems  involved  may  be  in  itself  mis- 
leading. Nevertheless,  the  success  which  is  attend- 
ing recent  aethereal  theories  of  material  phenomena 
seems  to  indicate  that  the  aether  has  in  truth 
properties  not  unlike  those  of  the  material  sub- 
stances with  which  we  are  acquainted. 

We  are  now  in  a  position  to  gather  together 
the  various  threads,  philosophical,  mathematical, 
and  experimental,  we  have  been  following  in 
this  and  the  preceding  chapters.  The  corpuscle 
of  J.  J.  Thomson,  the  electron  of  Stoney,  Larmor, 
and  Lorentz,  is  represented  in  the  aethereal  world 
by  Larmor's  conception  of  a  centre  of  intrinsic 
strain.  Unlike  the  vortex  atom,  this  strain-centre 
is  not  a  part  of  the  medium  for  ever  separated 
from  the  rest  ;  the  strain  alone  persists,  the 
part  of  the  aether  which  is  affected  by  it  con- 
stantly changes  as  the  sub-atom  is  moved.  The 
aether  is  stagnant,  and  the  sturdy  ghosts  which 
constitute  matter  float  to  and  fro  through  it  as 
waves  pass  over  the  surface  of  the  sea.  Such 
a  persistence  in  time  with  mobility  of  space 


280  PHYSICAL  SCIENCE 

would    be    impossible    for  a  strain-form    in    any 

elastic    solid    aether,  but    can  be    secured    by    a 

rotational   aether   of  the  type  described   by  Lord 
Kelvin. 

» 

According  to  this  view,  then,  an  electron  or 
unit  charge  of  electricity  is  a  centre  of  intrinsic 
strain,  probably  of  a  gyrostatic  type,  in  an 
aether,  which  is  also  the  medium  in  which  are 
propagated  the  waves  of  light  and  wireless  tele- 
graphy. Moreover,  the  electron  is  identical  with 
the  sub-atom  which  is  common  to  all  the  dif- 
ferent chemical  elements,  and  forms  the  universal 
basis  of  matter.  Matter,  at  any  rate  in  its  rela- 
tion to  other  matter  at  a  distance,  is  an  electrical 
manifestation  ;  and  electricity  is  a  state  of  in- 
trinsic strain  in  a  universal  medium.  It  is  prior 
to  matter,  and  therefore  not  expressible  in  terms 
of  matter  ;  it  is  sub-natural  if  not  super-natural. 

Thus,  from  the  side  of  aethereal  physics,  is 
reached  the  conception  of  an  electron  theory 
of  matter.  Within  the  last  few  years  experi- 
mental confirmations  of  the  fundamental  concep- 
tions of  that  theory  have  given  it  a  firmer 
position  than  could  be  hoped  at  the  time  the 
theory  was  formulated. 

The   property  of  mass,  the   most   fundamental 


ATOMS  AND  ^THER  281 

property  of  matter  for  dynamical  science,  is  ex- 
plained   by   the    electron   theory   as   an    effect   of 
electricity   in  motion.       Forasmuch   as  a  moving 
charge   carries   its   lines  of  electric  force  with  it, 
it    possesses    something    analogous    to    inertia    in 
virtue  of  its   motion.     The  quantitative  value  of 
this    effect    has    been    calculated    by    Thomson, 
Heaviside,    and     Searle.       Definite     experimental 
evidence  has  been  given  by  Kaufmann,  who  finds 
that   the    ratio    ejm    of    the    charge    to    the   mass 
for  the  corpuscles  ejected   by  radium  diminishes 
as  their  velocity  increases.     The  charge  is  almost 
certainly  constant,   and   thus   the   mass   must  in- 
crease with  the  velocity.     Theory  shows  that,  for 
a    slowly   moving   corpuscle,   the    electric    inertia 
outside  a  small  sphere  of  radius  a,  surrounding  the 
electrified  particle,  does  not  depend  on  the  velocity, 
and  is  measured  by  2e'2/^a  where  e  is  the  electric 
charge   on  the   particle.      But   when  the  velocity 
of  light  is   approached,  this  electric   mass  grows 
very   rapidly  ;    and,    on   the   assumption   that   the 
whole    of    the    mass    is    electrical,   Thomson   has 
calculated  the  ratio  of  the   mass  of   a   corpuscle 
moving   with   different   speeds   to   the   mass   of  a 
slowly    moving    corpuscle,    and    compared    these 
values    with    the    results    of    Kaufmann's    experi- 
ments. 


282 


PHYSICAL  SCIENCE 


Velocity  of  Corpuscle 
in  Centimetres  per  Second. 

Ratio  of  Mass  to  the  Mass  of  a 
slowly  moving  Corpuscle. 

Calculated. 

Observed. 

2.36  x  io10 
2.48  x  io10 
2.59  x  io10 
2.72  x  io10 
2.85  x  io10 

1.65 
1.83 
2.04 

2.43 
3-09 

i  '.66 

2.O 
2.42 
3-1 

In  this  remarkable  manner  has  it  been  possible 
to  obtain  experimental  confirmation  of  the  theory 
that  mass  is  an  electrical  phenomenon. 

To  explain  all  the  properties  with  which  we 
know  the  chemical  atoms  to  be  endowed,  and  more 
especially  their  power  of  complex  radiation,  it  is 
necessary  to  represent  an  atom  as  a  structure 
containing  a  large  number  of  electrons  in  steady 
orbital  motion  round  each  other,  somewhat  as 
the  planets  move  within  the  solar  system.  The 
attraction  of  gravity  is  independent  of  the  position 
of  a  body,  and  is  unaffected  by  any  kind  of  screen. 
It  would  be  difficult  to  explain  such  results  if  the 
electrons,  on  which  gravity  acts,  were  crowded 
together  ;  thus  it  seems  necessary  to  suppose  that 
the  electrons  occupy  an  exceedingly  small  fraction 
of  the  whole  volume  of  the  atom,  just  as  the 
planets  occupy  a  very  small  fraction  of  the  space 
comprised  within  their  orbits. 


ATOMS  AND  ^THER  283 

The  mass  of  the  electron  being  electrical  in  its 
nature,  we  may  calculate  the  size  of  the  individual 
electrons  or  corpuscles  from  the  expression  2e2/$a 
for  the  electrical  mass.  We  know  the  values  of 
e  and  of  e/nt,  and  from  these  results  we  calculate 
a  to  be  about  io~13  centimetre.  According  to 
Thomson,  a  is  the  radius  of  a  sphere  outside 
which  the  momentum  of  the  electric  field  exists. 
It  seems  reasonable  to  identify  this  sphere  with 
the  effective  dimensions  of  the  electron  itself. 

We  have  already  seen  that,  in  a  substance  like 
water,  where  the  molecules  are  fairly  closely  packed, 
one  cubic  centimetre  contains  about  3  x  ic22  mole- 
cules, or,  let  us  say,  io23  atoms.  Along  each  edge 
of  the  centimetre  cube  about  4  x  io7  atoms  are 
ranged,  and  thus  we  may  take  the  effective  radius 
of  an  atom  to  be  about  5  x  io~8  of  a  centimetre. 
Its  volume  would  be  about  io~23  of  a  cubic  centi- 
metre, while  the  volume  of  an  electron,  according 
to  the  above  estimate  of  the  radius,  is  about 
4  x  io39.  Thus,  while  the  diameter  of  an  electron 
is  less  than  the  hundred-thousandth  part  of  that 
of  an  atom,  the  volume  of  an  electron  is  only 
about  the  io~16  part  of  that  of  an  atom,  and 
their  relative  sizes  have  been  compared  by  the 
illustration  of  a  mouse  roaming  about  inside  a 
cathedral. 

On  the  planetary  theory  of  the  atom,  the  moving 


284  PHYSICAL  SCIENCE 

electric  charges  produce  a  magnetic  field,  just  as 
does  a  current  flowing  round  the  coils  of  a  galvano- 
meter. Thus,  conversely,  an  impressed  magnetic 
force  should  modify  the  movement  of  the  electrons, 
and  affect  their  radiation,  which  depends  on  the 
rate  of  acceleration  of  their  motion.  The  theory 
of  this  effect  was  considered  by  Lorentz  and 
Larmor,  who  predicted  the  subdivision  of  the 
spectral  lines,  afterwards  experimentally  discovered 
by  Zeeman. 

The  connection  of  the  electron  theory  with  the 
phenomena  of  radio-activity  has  already  been  con- 
sidered. The  conception  of  an  atom  as  a  system  of 
electrons  in  rapid  orbital  motion  naturally  suggests 
its  occasional  disintegration  ;  the  possibility  of  such 
disintegration  had  been  treated  as  a  difficulty  of 
the  theory  by  Larmor  before  the  discovery  of 
radio-activity  directly  indicated  its  occurrence. 
Some  attempts  have  recently  been  made  to  trace 
the  modus  operandi  of  atomic  disintegration. 

If  an  atom  consists  of  a  system  of  electrons  in 
orbital  movement,  the  acceleration  of  their  motion 
must  involve  a  constant  radiation  of  energy.  A 
single  negative  electron,  revolving  round  a  positive 
centre  of  greater  mass,  would  radiate  its  energy 
very  rapidly.  Two  electrons,  at  opposite  ends 
of  a  diameter,  would  radiate  much  more^  slowly, 
and,  as  the  number  of  electrons  increases,  the 


ATOMS  AND  AETHER  285 

rate  of  radiation  quickly  diminishes.  A  struc- 
ture like  an  atom,  with  its  mass-acceleration 
balanced,  would  maintain  its  energy  with  very 
little  loss  for  long  periods  of  time,  measured, 
perhaps,  in  millions  of  years.  Still,  loss  of  energy 
does  occur,  and  it  remains  to  consider  its  cumu- 
lative effects.  The  problem  for  one  or  a  small 
number  of  electrons  is  similar  to  that  given  by 
a  body  moving  in  a  circular  orbit,  retarded  by 
a  frictional  resistance,  a  problem  familiar  to 
ordinary  dynamics.  The  revolving  body  tends 
to  drift  towards  the  centre  of  its  orbit,  and  thereby 
the  attraction  towards  that  centre  will  clearly  in- 
crease its  velocity,  and  the  planetary  body  moves 
faster  and  faster  in  constantly  decreasing  orbits, 
and  might  finally  topple  over  into  a  new  configu- 
ration. If  the  velocity  of  light  were  approached, 
the  mass,  as  we  have  seen,  would  increase,  and 
it  has  been  argued  by  Lodge  that  this  sudden 
increase  in  mass  will  not  be  compensated  within 
the  system,  and  will  involve  a  sudden  gain  in 
momentum  which  may  result  in  an  explosive 
disintegration  of  the  atomic  system,  such  as 
occurs  in  the  phenomena  of  radio-activity.  It 
seems  likely,  however,  that,  when  the  increase  in 
mass  began  to  occur,  it  would  tend  to  check 
the  increase  in  velocity  which  produced  it,  and 
thus  fortunately  act  towards  stability.  Such  a 


286  PHYSICAL  SCIENCE 

transition  must  be  rapid ;  there  is  no  resting- 
place  between  one  atomic  configuration  and 
another. 

Another  account  of  possible  disintegration  has 
been  suggested  by  J.  J.  Thomson.  In  the  complex 
system  of  many  thousand  corpuscles,  constituting 
an  atom  of  a  heavy  chemical  element,  inter-corpus- 
cular forces,  as  well  as  those  acting  towards  the 
centre  of  the  system,  must  be  considered.  It  is 
probable  that  such  forces  would  counteract  any 
tendency  on  the  part  of  the  electrons  to  fall  with- 
out limit  towards  the  centre,  and  in  so  doing  to 
acquire  very  high  velocities  as  their  energy  was 
lost  by  radiation.  In  these  circumstances  the 
effect  of  radiation  is  to  diminish  the  velocity  of 
rotation.  The  rate  of  loss  of  energy,  as  we  said 
above,  is  very  slow ;  in  a  continuous  ring  of 
corpuscles  it  would  vanish,  but  still,  except  in 
special  circumstances,  with  a  number  of  separate 
corpuscles  a  slow  loss  is  always  going  on. 
Now  it  is  easy  to  conceive  a  system  of  revolving 
electrons,  stable  while  moving  fast,  which  be- 
comes unstable  when  the  velocity  of  rotation 
sinks  to  a  critical  value.  For  instance,  as  we 
have  already  done  above,  let  us  represent  the 
structure  of  an  atom  by  a  number  of  little 
magnets  thrust  through  corks,  floating  on  the 
surface  of  water  with  their  south  poles  vertically 


ATOMS  AND  .ETHER  287 

upwards.  Let  the  north  pole  of  a  large  bar 
magnet  be  placed  above  the  water.  It  will  attract 
the  south  poles  of  the  little  floating  magnets,  which 
repel  each  other,  and  thus  draw  them  together  till 
the  repulsive  forces  balance  the  attractive  ones.  To 
take  a  very  simple  case,  suppose  that  we  had  six 
little  floating  magnets,  and  that  we  gave  the  water 
and  the  magnets  floating  on  it  a  circular  movement 
so  that  the  six  little  magnets  revolved  round  the 
centre.  If  the  movement  were  fast  enough,  the 
six  magnets  would  place  themselves  all  on  the 
circumference  of  a  circle  at  equal  distances  from 
each  other  ;  and  this  arrangement  is  permanent  as 
long  as  the  velocity  of  rotation  remains  above  a 
certain  value.  Presently,  however,  the  velocity 
diminishes  to  such  an  extent  that  the  configuration 
becomes  unstable,  and  a  sudden  rearrangement 
occurs.  The  magnets  place  themselves  five  on 
the  circumference  of  the  circle,  and  one  at  the 
centre,  in  the  positions  they  assume  when  the 
whole  system  is  at  rest.  Such  a  model  may  be 
crude,  but  it  is  strikingly  effective.  As  we  have 
seen  above,  Thomson  has  correlated  it  with  the 
system  of  corpuscles  revolving  within  a  sphere  of 
uniform  positive  electrification,  and,  in  this  case 
also,  has  shown  that  similar  changes  of  con- 
figuration must  follow  a  diminution  of  velocity. 
In  the  complex  system  constituting  an  atom,  it  is 


288  PHYSICAL  SCIENCE 

easy  to  imagine  that  such  a  sudden  change  in  the 
conditions  of  stability  might  well  result  in  an  ex- 
plosive rearrangement,  in  which  the  atom  might  be 
shivered  to  pieces,  and  give  rise  to  the  phenomena 
of  radio-activity. 

A  more  fundamental  suggestion  has  been  made 
by  J.  H.  Jeans,  who  imagines  that  radio-activity 
may  result  from  the  coalescence  of  positive  and 
negative  electrons.  On  this  idea  the  energy  of 
radio-active  atoms  is  supplied  by  the  actual 
destruction  of  matter. 

J.  J.  Thomson  has  also  described  a  process  of  con- 
tinual evolution  and  disintegration  of  matter.  The 
primordial  chaos  is  filled  with  corpuscles,  in  posi- 
tive and  negative  pairs,  scattered  throughout  space. 
By  mutual  forces,  due  to  the  accompanying  strains 
in  the  aether,  they  attract  each  other,  and  produce 
mutual  accelerations.  Two  doublets  meeting  with 
very  high  velocities  will  probably  part  again  after 
a  merely  hyperbolic  acquaintance,  but  the  mutual 
accelerations  imply  loss  of  energy  by  radiation, 
and  thus  the  average  velocity  will  be  reduced  till 
occasionally  permanent  connections  between  cor- 
puscular doublets  may  be  formed.  Gradually 
these  systems  grow  in  size  and  complexity,  and  the 
chemical  atoms  are  evolved,  many  of  their  periodic 
relations  being  represented  in  a  wonderful  manner 
by  the  crude  model  of  systems  of  floating  magnets 


ATOMS  AND  AETHER  289 

already  described.  But  none  of  these  atoms  are 
dead  ;  they  all  possess  corpuscular  motion,  and, 
as  radiation  produces  loss  of  energy,  occasional 
positions  of  instability  are  reached  at  which  re- 
arrangement is  necessary.  Such  positions  will 
occur  more  frequently  in  complicated  structures, 
and  thus  it  is  natural  that  radio-activity  is  more 
marked  in  the  heavier  atoms. 

Darwin  and  Wallace  revealed  to  us  the  evolution 
of  living  organisms  ;  it  seems  possible  that  Thom- 
son, Larmor,  and  Rutherford  may  enable  us  to 
trace  the  corresponding  process  in  inorganic 
matter. 

One  of  the  most  remarkable  of  the  purely 
mechanical  models  of  the  aether  has  been  de- 
scribed recently  by  Professor  Osborne  Reynolds 
in  his  treatise  entitled,  "  The  Sub-Mechanics  of 
the  Universe."  In  this  scheme  the  aethereal-strain 
theory  of  matter  has  been  formulated  in  a  definite 
mechanical  manner,  and  mobility  for  the  strain- 
form  secured  by  other  than  gyrostatic  conceptions 
of  the  medium.  It  is  claimed  that  lt  there  is  one, 
and  only  one  conceivable  purely  mechanical  system 
capable  of  accounting  for  all  the  physical  evidence, 
as  we  know  it,  of  the  universe.  The  system  is 
neither  more  nor  less  than  an  arrangement,  of 
indefinite  extent,  of  uniform  spherical  grains 

T 


290  PHYSICAL  SCIENCE 

generally  in  normal  piling  so  close  that  the 
grains  cannot  change  their  neighbours,  although 
continually  in  relative  motion  with  each  other  ; 
the  grains  being  of  changeless  shape  and  size." 
The  grains  are  of  minute  size,  small  even  com- 
pared with  Thomson's  corpuscles,  their  diameter 
being  5.534  x  io~18  centimetre.  The  pressure  in 
the  medium  is  about  10,000  tons  per  square  centi- 
metre. 

"  In  spaces  in  which  there  are  local  inequalities 
in  the  medium  about  local  centres,  owing  to  the 
absence  or  presence  of  a  number  of  grains  in 
deficiency  or  excess  of  the  number  necessary  to 
render  the  piling  normal,  such  local  inequalities 
are  permanent,"  they  can  move  through  the 
medium,  though  the  grains  do  not  move  with 
them.  The  excess  or  deficiency  passes  along,  as 
a  wave  passes  over  water.  The  form  persists, 
though  the  substance  does  not  move  with  it. 
Positive  inequalities,  if  such  exist,  repel  each 
other,  and  would  thus  pass  away  as  far  from 
each  other  as  possible,  but  negative  inequalities 
attract  each  other  according  to  the  laws  of  .gravi- 
tation, and  constitute  the  particles  of  matter. 
Matter,  then,  is  constituted  by  regions  of  dimi- 
nished mass,  and  this  result  led  Reynolds  to  call 
his  popular  exposition  of  the  theory  "  An  Inver- 


ATOMS  AND  AETHER  291 

sion  of  Ideas  regarding  the  Structure  of  the 
Universe." 

Reynolds  shows  that  these  assumptions  are 
consistent  with  the  known  numerical  values  of 
the  gravitation  constant  and  the  velocity  of 
light.  Explanations  are  also  given  of  numer- 
ous other  physical  phenomena,  optical,  electri- 
cal, &c. 

The  mathematical  analysis  by  which  these 
deductions  are  established  is  very  complex  and 
difficult,  and  it  is  yet  too  soon  to  say  if  this  bold 
attempt  will  stand  the  criticisms  that  will  be 
directed  towards  it ;  but  Professor  Osborne 
Reynolds'  great  reputation,  and  the  twenty  years 
he  has  laboured  at  this  research,  will  ensure  for  it 
a  careful  consideration  from  those  competent  to 
judge  of  its  merits. 

Should  this,  or  any  similar  theory,  stand  the 
test  of  time,  a  mechanical  specification  of  the 
universe  will,  in  one  sense,  have  been  obtained. 
But  it  is  evident  that  the  success  of  such  theories 
does  but  shift  the  mystery  of  the  unknown. 
Matter  is  a  persistent  strain-form  flitting  through 
a  universal  sea  of  aether  :  we  have  explained 
matter  in  terms  of  aether.  JEther  in  its  turn  is 
described  as  a  fairly  close-packed  conglomerate  of 
minute  grains  in  continual  oscillation  :  we  have 


292  PHYSICAL  SCIENCE 

explained  the  properties  of  the  aether.  So  be  it. 
But  what  of  the  grains  of  which  the  aether  is  com- 
posed ?  Are  they  "  strong  in  solid  singleness,"  like 
the  one-time  atoms  of  Lucretius  ?  Or  have  they 
parts,  within  which  opens  a  new  field  of  com- 
plexity ?  Of  what  substance  are  they  made  ?  Has 
a  new  aether  more  subtle  than  the  first  to  be 
invoked  to  explain  their  properties,  and  a  third 
aether  to  explain  the  second  ?  The  mind  refuses 
to  rest  content  at  any  step  in  the  process.  An 
ultimate  explanation  of  the  simplest  fact  remains, 
apparently  for  ever,  unattainable. 


CHAPTER     VIII 

ASTRO-PHYSICS 

"  For  who  so  list  into  the  heavens  looke, 
And  search  the  courses  of  the  rowling  spheares, 
Shall  find  that  from  the  point  where  first  they  tooke 
Their  setting  forth,  in  these  few  thousand  yeares 
They  all  are  wandred  much  ;  that  plaine  appeares  : 

Ne  is  that  same  great  glorious  lampe  of  light, 
That  doth  enlumine  all  these  lesser  fyres, 
In  better  case,  ne  keepes  his  course  more  right, 
But  is  miscaried  with  the  other  Spheres  : 
For  since  the  terme  of  fourteene  hundred  yeres, 
That  learned  Ptolomae  his  hight  did  take, 
He  is  declyned  from  that  marke  of  theirs 
Nigh  thirtie  minutes  to  the  Southern  lake ; 
That  makes  me  feare  in  time  he  will  us  quite  forsake." 
—SPENSER,  The  Faerie  Queene,  Book  V. 

THE  origins  of  the  ancient  science  of  astronomy 
are  lost  in  the  mists  of  the  past.  Unlike  some  of 
the  subjects  we  have  discussed  in  this  volume, 
its  phenomena  are  familiar  to  the  most  unob- 
servant of  mankind,  and  some  of  these  pheno- 
mena, in  the  apparently  unfailing  regularity  of 
their  manifestation,  have  served  as  measurers  of 
time  and  forewarners  of  seasons  during  imme- 
morial ages. 

293 


294  PHYSICAL  SCIENCE 

The  recognition  of  the  possibility  of  slow  change 
in  this  regularity,  and  the  attempt  to  detect  such 
change  by  careful  observation,  are  also  an  old  story, 
while  unusual  manifestations,  such  as  comets  and 
eclipses,  were,  till  comparatively  recent  times, 
regarded  with  fear  and  consternation,  and  con- 
sidered as  direct  signs  of  Divine  wrath. 

Yet  the  oldest  of  the  sciences  is  also,  in  some 
respects,  if  not  the  newest,  at  any  rate  among  the 
youngest  of  the  fraternity;  for  in  its  recent  growth, 
its  spirit  of  adventure,  its  capacity  of  immediate 
development,  it  shows  all  the  characteristics  of 
sturdy  youth. 

In  the  history  of  the  different  branches  of 
physical  science,  it  is  constantly  found  that 
periods  of  great  activity  and  advancing  know- 
ledge alternate  with  periods  when,  owing  to  the 
exhaustion  of  the  possibilities  of  the  apparatus 
available  or  of  the  methods  of  research  em- 
ployed, progress  seems  almost  to  cease. 

Fifty  years  ago  astronomy  appeared  to  be 
sinking  into  one  of  these  periods  of  compara- 
tive stagnation.  The  power  of  the  telescope 
seemed  almost  to  have  reached  a  limit,  for 
although  improved  and  larger  instruments  were 
continually  being  produced,  the  revelations  they 
made  were  apparently  unworthy  of  the  knowledge 
and  skill  lavished  on  their  manufacture.  It  was 


ASTRO-PHYSICS  295 

not  more  elaborate  instruments,  but  new  methods 
of  research  that  were  wanting. 

But  even  while  the  older  astronomy  was  flag- 
ging, the  new  method  had  appeared,  and  was  only 
waiting  for  development  in  its  apparatus  to  carry 
forward  the  torch  of  learning  into  untrodden 
paths,  and  even  to  rival  the  results  of  Adams 
and  Leverrier,  who  had  stirred  so  profoundly 
the  imagination  of  their  generation. 

The  new  science  of  astro-physics  dates  from 
the  application  of  the  spectroscope  to  astrono- 
mical problems.  The  spectroscope  itself  illus- 
trates the  progressive  triumph  of  modern  science, 
for  it  is  the  work  neither  of  one  man  nor  of 
one  century.  Its  principles  have  been  gradually 
developed  and  its  construction  elaborated  through- 
out a  couple  of  hundred  years.  Newton  was 
the  first  to  analyse  the  light  of  the  sun  by  a 
prism,  to  study  the  spectrum  thus  obtained,  and 
to  show  that  it  consists  of  rays  of  every  colour, 
which,  when  blended  together  in  the  eye,  pro- 
duce the  sensation  of  white  light.  In  the  year 
1802,  Wollaston  noticed  that  the  spectrum  of  the 
sun's  light  was  crossed  by  a  number  of  fine  dark 
lines,  and  the  relative  positions  of  these  lines  were 
shortly  afterwards  carefully  mapped  by  Fraunhofer, 
whose  name  the  lines  have  borne  since  that  time. 


296  PHYSICAL  SCIENCE 

The  next  great  advance  was  made  by  the 
chemists  Bunsen  and  Kirchoff,  who  repeated  and 
amplified,  in  the  year  1860,  an  almost  forgotten 
experiment  of  Foucault,  though  the  principles 
which  underlie  their  discovery  had  previously 
been  understood  by  Sir  George  Stokes.  Any 
vibrating  system — a  child's  swing,  for  example- 
is  set  into  violent  oscillation  if  impulses  are 
given  to  it  exactly  timed  to  coincide  with  its 
own  proper  period  of  vibration.  Just  as  the 
particular  piano  wires  which  are  tuned  to  a  par- 
ticular note  will  be  set  in  vibration  when  that 
note  is  sounded  in  their  neighbourhood,  so  the 
molecules  or  atoms  of  a  gas  will  be  set  in  vibra- 
tion by  waves  of  light  which  possess  a  period 
of  oscillation  corresponding  to  their  own.  A 
complex  wave  of  light,  then,  passing  through  a 
collection  of  such  molecules  or  atoms,  will  have 
those  constituent  waves  absorbed  which  are  tuned 
to  the  characteristic  periods  of  the  absorbing 
systems.  Substances,  that  is  to  say,  absorb  the 
particular  kinds  of  radiation  which  they  would 
themselves  emit  when  hot. 

Applying  these  principles  to  the  Fraunhofer 
lines,  Stokes  held  that  when  coincidences  existed 
between  their  positions  and  those  of  the  bright 
lines  obtained  by  examining  with  a  prism  the 
light  of  incandescent  vapours,  the  coincidence 


ASTRO-PHYSICS  297 

was  to  be  interpreted  by  the  supposition  that 
similar  vapours  were  present  in  the  atmosphere 
of  the  sun,  and  absorbed  the  light  coming  from 
the  hotter  regions  below  them. 

In  1860  Bunsen  and  KirchofT,  without  knowing 
that  Foucault  had  anticipated  them  in  1849, 
devised  and  carried  out  an  experiment  on  the 
artificial  production  of  Fraunhofer  lines.  They 
passed  the  light  of  an  electric  arc,  which  gave 
a  perfectly  continuous  spectrum  with  no  such 
lines  as  those  in  the  solar  light,  through  the 
vapour  of  sodium  volatilised  in  the  comparatively 
cool  region  of  a  spirit  lamp  flame.  They  had 
the  joy  of  seeing  a  black  absorption  line,  co- 
incident with  the  bright  line  given  by  hot  sodium 
vapour,  crossing  the  continuous  spectrum  of 
the  arc,  just  as  the  black  line,  called  by  Fraun- 
hofer the  line  D,  crosses  the  spectrum  of  the 
sun.  The  possibility  of  determining  the  chemical 
constitution  of  the  heavenly  bodies  had  opened 
before  the  eyes  of  man. 

Hitherto  the  sun  had  been  studied  chiefly  in 
relation  to  the  earth  and  the  general  solar  system, 
while  little  else  was  known  about  the  stars  than 
their  apparent  relative  positions  on  a  hypothetical 
celestial  sphere.  Their  composition  and  physical 
condition  were  held  to  be  outside  the  range  of 
any  definite  scientific  investigation;  subjects,  per- 


298  PHYSICAL  SCIENCE 

haps,  better  fitted  to  the  romancer  than  to  the 
serious  student.  But  with  the  advent  of  the 
spectroscope,  sun  and  stars,  in  a  new  aspect, 
re-entered  the  realm  of  exact  knowledge,  and 
began  to  give  up  the  secrets  of  their  composition 
and  state. 

Many  of  the  chemical  elements  known  on  the 
earth  were  detected  in  the  sun,  while  Fraun- 
hofer  lines,  not  corresponding  to  the  spectrum 
of  any  terrestrial  substance,  suggested  the 
existence  of  solar  elements  hitherto  unrecog- 
nised. The  spectra  of  the  stars  were  found  to 
vary,  some  showing  the  presence  of  hydrogen 
only,  while  others  indicated  the  existence  of 
constitutions  more  nearly  approaching  that  of 
our  sun. 

The  structure  of  the  nebulae,  those  vast,  vague 
sources  of  luminosity,  had  long  been  a  matter 
of  speculation.  Were  they  clusters  of  innumer- 
able stars,  so  minute  and  so  distant  that  the 
most  powerful  telescopes  could  not  resolve  them, 
or  were  they,  indeed,  as  their  name  indicated, 
foregatherings  of  cloud-like,  light-giving  vapours? 
The  question  was  settled  as  soon  as  the  spectro- 
scope was  turned  towards  their  light.  A  con- 
tinuous gradation  in  properties  was  found  between 
stars  and  nebulae.  Most  nebulae  gave  continuous 
spectra,  indicating  high  density  and  pressure  at 


ASTRO-PHYSICS  299 

the  source  of  radiation,  but  some  few  gave 
bright  lines  on  a  dark  background — the  spectra, 
not  of  dense  suns  surrounded  by  cooler  atmos- 
pheres, but  of  masses  of  glowing  vapour  of 
great  tenuity — the  beginnings,  perhaps,  of  suns 
and  worlds  yet  to  be. 

Then  came  a  pause  in  the  progress  of  this 
new  branch  of  knowledge.  The  spectroscope 
alone  seemed  to  have  told  all  it  could  to  the 
human  eye.  A  more  sensitive  instrument  was 
needed  to  receive  its  messages,  to  intensify  them, 
and  to  interpret  them  to  the  senses  of  mankind. 
It  was  not  till  photography  was  employed  to 
record  the  results  of  spectrum  analysis  that  the 
full  power  of  the  spectroscope  was  understood. 
Although  previous  attempts  had  been  made  by 
means  of  inferior  processes  to  photograph  the 
spectra  of  the  sun  and  stars,  the  great  success 
of  the  method  dates  from  the  application  of  the 
dry  gelatine  process  by  Sir  William  Huggins  in 
1876 

The  photographic  method  has  many  advan- 
tages over  direct  visual  observation.  The  sensitive 
plate  can  be  exposed  for  a  considerable  length 
of  time,  -and  the  effect  of  the  light  on  it  is 
cumulative.  Excessively  feeble  light  will,  by  pro- 
longed action,  produce  a  sensible  impression  on 
the  photographic  plate  when  it  would  be  quite 


300  PHYSICAL  SCIENCE 

insensible  to  the  eye,  which  has  none  of  this 
power  of  gradually  storing  and  intensifying  its 
impressions.  Again,  the  photograph  will  record 
ultra-violet  radiation  to  which  the  nerves  of  the 
eye  do  not  respond,  and,  in  this  way,  it  has 
revealed  many  invisible  lines.  Finally,  the  photo- 
graph forms  a  permanent  record,  to  which  re- 
ference can  be  made  at  any  future  time,  and 
permits  measurements,  more  accurate  than  those 
made  by  direct  visual  observation,  to  be  obtained 
at  leisure  in  the  laboratory  many  hours  or  days 
after  the  exposure.  In  several  observatories,  sys- 
tematic records  are  kept  of  the  state  of  the  sky 
from  night  to  night,  and,  more  than  once,  when 
a  new  star  has  been  detected,  its  previous  history 
has  been  unfolded  by  reference  to  photographic 
plates  exposed  before  the  existence  of  the  new 
star  was  suspected. 

Two  methods  of  obtaining  spectra  are  known 
to  the  physicist,  the  instruments  used  being 
respectively  the  prism  and  the  grating.  The 
grating  consist  of  a  number  of  equidistant 
parallel  scratches  ruled  on  a  reflecting  surface 
of  polished  metal  or  on  a  transparent  surface  of 
glass.  The  scratches  are  very  close  together, 
many  thousands  of  them  being  included  in  the 
space  of  an  inch.  When  a  wave  of  light  falls  on 
a  metallic  grating,  the  scratches  refuse  to  reflect 


ASTRO-PHYSICS  301 

the  light.  The  distances  between  the  scratches  are 
comparable  with  the  minute  wave-lengths  of  light, 
and  thus  different  waves  are  differently  treated  by 
the  grating.  The  component  rays  of  a  complex 
beam  of  light  are  separated  from  each  other,  and, 
if  the  source  of  light  be  a  narrow  slit,  a  number 
of  parallel  images  are  formed,  and  a  spectrum  is 
obtained.  The  deviation  of  any  particular  wave, 
such  as  the  yellow  sodium  ray,  will  obviously 
depend  on  the  wave-length  of  the  light,  and,  for 
the  same  grating,  will  depend  on  this  wave-length 
alone.  The  spectral  lines  obtained  will  therefore 
have  positions  simply  depending  on  the  wave- 
length or  period  of  vibration  of  the  correspond- 
ing rays  of  light ;  in  this  differing  from  the  similar 
lines  given  by  the  prism,  which  depend  in  position 
on  the  qualities  of  the  glass  as  well  as  on  the 
periodic  times  of  vibration  of  the  different  rays. 

The  sharpness  of  definition  of  a  spectrum  taken 
from  a  grating  depends  on  the  accuracy  with  which 
the  scratches  are  ruled,  and  thus  the  perfection  of 
the  grating  depends  on  our  power  of  moving  the 
scratching  tool  through  exactly  equal  intervals 
between  two  scratches.  To  control  the  move- 
ment a  perfect  screw  is  required,  and  to  Professor 
Rowland's  improvement  in  the  manufacture  of 
screws  in  1882,  and  to  his  idea  of  using  them  to 
rule  gratings  on  concave  metallic  surfaces,  is  directly 


302  PHYSICAL  SCIENCE 

due  the  possibility  of  making  adequate  use  of  the 
resources  of  photography  in  the  province  of  solar 
and  stellar  spectrum  analysis.  The  arts  and  the 
sciences  are  closely  related  ;  an  advance  in  one 
of  them  often  leads  to  a  corresponding  advance  in 
the  other,  and  it  is  not  always  science  that  leads 
the  way. 

The  concave  grating  banished  the  need  for  a 
lens  to  focus  the  rays  after  diffraction,  and  an 
image  of  the  spectrum  could  now  be  obtained 
from  the  grating  alone.  Glass  is  opaque  to  much 
of  the  ultra-violet  radiation,  in  which  sunlight,  at 
any  rate,  is  very  rich.  Prismatic  spectra  and 
spectra  taken  with  plane  gratings  and  lenses  do 
not  show  the  ultra-violet  lines.  But,  by  the  use  of 
a  concave  grating  and  a  reflecting  telescope,  the 
presence  of  glass  becomes  unnecessary,  and  in- 
vestigation can  be  prolonged  into  the  ultra-violet 
region  till  the  increasing  absorption  of  the  earth's 
atmosphere  prevents  the  rays  from  reaching  the 
surface  of  the  ground. 

Glass  is  opaque  to  the  infra-red  radiation  also, 
and  here  again  the  advantages  of  the  concave 
grating  are  manifest.  The  infra-red  spectrum  has 
been  examined,  chiefly  by  Professor  S.  P.  Langley  . 
of  Washington,  through  the  heating  effects  of  its 
constituent  rays.  Professor  Langley  uses  an  instru- 
ment called  the  bolometer,  in  which  the  heating 


ASTRO-PHYSICS  303 

effects  of  different  parts  of  the  spectrum,  and  con- 
sequently the  position  of  the  dark  lines,  are  deter- 
mined by  measuring  the  change  in  electric  resistance 
of  a  very  thin  strip  of  platinum  exposed  to  the 
radiation.  This  form  of  platinum  thermometer  is 
extremely  sensitive,  and  the  spectrum  of  the  sun 
has  been  mapped  far  below  the  limits  within 
which  the  eye  responds  to  the  stimulus  of  light. 

Perhaps  the  most  striking  and  interesting  results 
given  by  the  combination  of  camera  and  spectro- 
scope are  those  obtained  by  the  determination  of 
the  change  in  the  refrangibility  of  light  produced 
by  relative  motion  of  approach  or  retrocession  of 
the  source  of  light  and  the  receiving  station.  Let 
us  imagine  that  waves  are  proceeding  from  some 
source  which  remains  at  rest.  A  certain  number 
of  waves  reach  an  observer  in  one  second.  If, 
however,  the  observer  is  approaching  the  source, 
it  is  evident  that,  as  he  is  going  to  meet  the  waves, 
a  greater  number  of  them  will  reach  him  in  one 
second  than  when  he  was  at  rest.  Similarly,  if 
the  observer  move  away  from  the  source,  the 
number  of  waves  which  reach  him  in  a  given 
time  will  be  less  than  before.  The  same  effects 
will  be  produced  if  the  observer  be  stationary  and 
the  source  of  light  move.  Doppler's  principle,  as 
this  change  in  periodic  time  is  called,  is  well  illus- 


304  PHYSICAL  SCIENCE 

trated  in  the  case  of  sound.  Here  the  frequency 
of  wave  impulse  on  the  ear  determines  the  pitch 
of  the  note  heard,  and  it  is  easy  to  detect  a  distinct 
flattening  by  a  semitone  or  more,  as  the  whistling 
engine  of  an  express  train  passes  the  observer. 
The  source  of  the  waves  of  sound  still  vibrates 
with  the  same  frequency,  the  change  is  only  in 
the  number  of  impulses  reaching  the  observer  per 
second. 

The  frequency  with  which  waves  of  light  are 
received  by  the  optic  nerve  determines  the  colour 
perceived  by  the  brain,  and  also  the  amount  of 
refraction  in  passing  through  a  prism.  Thus  the 
colour  of  a  ray  of  a  single  definite  wave-length, 
as  well  as  its  position  in  the  prismatic  spectrum, 
will  be  different  from  the  normal  value  when  the 
source  of  light  and  the  observer  are  moving  re- 
latively to  each  other.  An  approach  will  result 
in  a  shifting  towards  the  blue  end  of  the  spectrum 
owing  to  the  increase  in  frequency  ;  a  recession 
will  involve  a  reddening  of  the  light,  or  a  move- 
ment of  the  spectral  lines  towards  the  red  end  of 
the  spectrum.  Owing  to  the  great  velocity  of 
light,  the  change  will  relatively  be  much  less  than 
in  the  case  of  sound.  Light  travels  about  186,000 
miles  in  one  second,  and,  great  though  the  speeds 
of  the  stars  may  be,  they  fall  far  short  of  such 
tremendous  values.  A  velocity  of  eighteen  miles  a 


ASTRO-PHYSICS  305 

second,  for  example,  the  velocity  of  the  earth  in 
her  orbit,  is  but  the  ten-thousandth  part  of  the 
velocity  of  light.  This  velocity  of  approach,  then, 
would  involve  a  change  of  the  ten-thousandth  part 
in  the  period  of  vibration  of  the  incident  light.  The 
whole  visible  spectrum,  from  the  red  to  the  violet 
of  the  rainbow,  includes  a  range  of  frequencies  of 
about  an  octave,  that  is,  the  period  of  vibration  of 
the  extreme  red  is  about  double  that  of  the  extreme 
violet.  A  velocity  equal  to  that  of  the  earth,  then, 
would  involve  a  change  in  position  of  a  spectral 
line  of  about  the  five-thousandth  part  of  the 
total  length  of  the  spectrum.  Many  stars  are 
approaching  or  receding  from  the  earth  at  velo- 
cities higher  than  that  which  we  have  taken  as 
an  example,  but  still  the  changes  in  position  to 
be  measured  are  very  small,  and  refined  methods 
and  great  experimental  skill  are  needed  for  ac- 
curate results. 

The  problem  of  determining  the  movement  of  a 
star  travelling  along  the  straight  line  joining  it  to 
the  observer  would,  before  this  principle  was  dis- 
covered, have  seemed  one  of  the  most  hopeless 
problems  which  a  cynical  scientific  sceptic  could 
propose  for  solution  to  the  physicist.  Yet  such 
problems  are  now  solved  daily,  or  rather  nightly  ; 
solved,  indeed,  much  more  readily  than  they  could 

U 


3o6  PHYSICAL  SCIENCE 

be  if  the  star  were  moving  across  the  line  of  sight. 
In  the  latter  case,  even  if  a  knowledge  of  the  dis- 
tance makes  the  determination  possible,  prolonged 
observations  are  needed,  extending  over  months 
or  years,  till  the  movement  becomes  apparent  at 
the  distance  of  the  earth.  Many  stars  are  so 
distant  that  no  such  cross  movement  could  be 
detected  in  any  reasonable  time.  If,  however, 
the  star  is  moving  towards  or  away  from  the 
earth,  the  spectroscope  is  turned  towards  it,  and 
in  the  short  time  required  to  fix  a  photographic 
impression,  develop  and  print  the  plate  and 
measure  the  lines  upon  it,  the  velocity  of  the  star 
can  be  determined. 

Another  application  of  the  same  principle  has 
enabled  us  to  demonstrate  directly  the  rotation  of 
the  sun  on  its  axis,  and  to  separate  those  absorption 
lines  in  the  spectrum  of  the  sun's  light  which  are 
due  to  the  effect  of  the  earth's  atmosphere  from 
the  lines  of  true  solar  origin.  One  limb  of  the 
sun  is,  at  any  moment,  approaching  the  earth, 
while  the  opposite  limb  is  in  like  manner  reced- 
ing. By  pointing  a  spectroscope  first  at  one  limb 
and  then  at  the  other,  a  shift  of  the  spectral  lines 
is  seen  ;  and,  from  the  amount  of  the  displace- 
ment, the  velocity  of  movement  of  the  glowing 
gases  which  produce  the  lines  of  absorption  can 
be  calculated.  Lines  which  are  not  shifted  by 


FIG.  36.— C  LINE  IN  THE  SPECTRUM  OF  A  SUN-SPOT 
(Professor  Hale) 

To  face  page  307 


ASTRO-PHYSICS  307 

this  operation  are  clearly  not  of  solar  origin,  and 
are  consequently  to  be  referred  to  absorption  by 
the  atmosphere  of  the  earth. 

Other  problems  in  solar  physics  have  been 
solved  by  the  same  method.  The  existence  of 
sun  spots  has  long  been  known  ;  they  were, 
indeed,  familiar  to  the  Chinese  in  very  early 
times,  and,  in  the  middle  of  the  nineteenth  cen- 
tury, their  periodic  increase  and  decrease  in  a 
cycle  of  ten  or  eleven  years  was  noted  by  western 
observers,  and  a  coincident  period  of  terrestrial 
magnetic  phenomena  was  established.  The  struc- 
ture and  properties  of  sun  spots  were  then  seen 
to  possess  more  than  a  local  solar  interest,  and 
their  importance  with  regard  to  terrestrial  me- 
teorology became  manifest.  It  has  long  been 
held  that  sun  spots  were  the  seat  of  move- 
ments of  gases  on  a  gigantic  scale  in  the  solar 
atmosphere,  and  direct  evidence  of  such  storms  is 
supplied  by  the  spectroscope.  Professor  Hale  gives 
a  drawing  of  the  spectrum  of  a  sun  spot  in  the 
neighbourhood  of  the  C  line.  This  drawing  is 
reproduced  in  Fig.  36.  The  slit  of  the  spectro- 
scope was  directed  to  the  sun's  disc  so  as  to 
include  the  area  covered  by  the  spot.  The  figure 
shows  a  small  part  of  the  spectrum,  which  extends 
from  left  to  right  across  the  paper.  The  faint 
horizontal  dark  line  shows  the  effect  of  the  sun 


308  PHYSICAL  SCIENCE 

spot,  from  which  much  less  light  proceeds  than 
from  the  rest  of  the  sun's  surface.  Several  faint 
Fraunhofer  lines  cross  the  diagram  vertically,  and 
it  will  be  seen  that  these  lines  are  still  dark  lines 
in  the  sun  spot  region.  The  sun  spot  itself,  then, 
must  be  the  source  of  continuous  radiation,  from 
which  definite  rays  are  abstracted  by  cooler  gases 
in  higher  regions,  the  process  being  identical  with 
that  going  on  in  other  parts  of  the  sun.  The 
heavy  dark  line  crossing  the  figure  from  top  to 
bottom  is  the  C  line  to  which  reference  has  been 
made.  It  is  much  stronger  and  darker  than  any 
of  the  other  lines  shown.  In  the  neighbourhood 
of  the  sun  spot,  like  the  fainter  lines,  it  still  shows 
dark,  but  in  its  centre  is  a  bright  patch  or  reversal 
of  the  line.  This  intense  luminosity  indicates  that, 
superposed  on  the  layer  of  gas  which  absorbs  the 
light,  is  a  mass  so  hot  that  its  radiation  is  even 
greater  than  the  normal  radiation  from  the  sun's 
surface.  The  curious  hook-like  appendage  to  the 
line,  which  begins  as  a  fine  point  in  the  middle  of 
the  sun  spot  absorption,  and  ends  above  by  fusing 
with  the  C  line,  tells  of  an  extraordinary  outrush 
of  cool  hydrogen  coming  from  the  centre  of  the 
sun  spot  area,  and  travelling  outwards  with  a 
radial  velocity  of  about  one  hundred  and  twenty 
miles  a  second.  In  its  outward  course  it  passes 
away  from  the  sun  spot  area,  and  finally  comes  to 


ASTRO-PHYSICS  309 

rest  at  a  distance  of  thirty  to  forty  thousand  miles 
from  its  point  of  origin.  Its  absorption  then  of 
course  coincides  in  spectral  position  with  the 
normal  C  line. 

Similar  work,  carried  on  in  several  observa- 
tories, has  thrown  much  light  on  the  movements 
of  the  prominences,  which  come  into  view  at  the 
edge  of  the  sun's  disc,  and  seem  to  be  connected 
intimately  with  the  spots.  These  enormous  masses 
of  glowing  gases  produce  bright  line  spectra,  and 
the  displacement  of  the  lines  gives  the  movement 
in  one  plane,  while  direct  visual  observation  gives 
that  in  a  plane  at  right  angles  to  the  first.  Thus 
the  motion  of  the  prominences  can  be  completely 
specified.  Their  velocities  are  often  as  high  as 
200  or  300  miles  a  second. 

It  seems  unlikely  that  such  high  velocities  can 
be  the  result  of  differences  of  gaseous  pressure  and 
the  convection  currents  thus  engendered.  They 
are  more  probably  to  be  explained  by  the  local 
action  of  some  explosive  source  of  energy,  by 
which  matter  is  projected  with  great  violence. 

The  application  of  Doppler's  principle  to  stellar 
movement  has  led  to  other  results  quite  as  re- 
markable as  those  already  described.  Our  sun 
is  a  single  system,  but  many  of  his  fellows  among 
the  stars  are  accompanied  by  partners  ;  the  two 


310  PHYSICAL  SCIENCE 

existing  in  more  or  less  close  conjunction,  and 
showing  all  the  signs  of  a  common  origin. 
Some  of  these  double  stars  can  be  examined 
by  telescopic  means,  but  the  majority  of  them 
lie  too  close  together  to  be  thus  separated. 
Often,  too,  one  of  the  pair  is  not  luminous,  and 
therefore  would  never  be  visible.  In  this  class 
are  probably  to  be  placed  variable  stars,  such  as 
the  star  known  as  Algol  or  f$  Persei,  the  light 
from  which  undergoes  periodical  fluctuations  in 
intensity.  The  light  keeps  constant  for  the 
greater  part  of  the  cycle,  and  then  diminishes 
for  a  short  time  before  again  rising  to  its  normal 
value.  This  behaviour  was  long  suspected  to  be 
due  to  the  partial  concealment  of  the  star  by  a 
dark  companion  or  satellite,  and  the  surmise  was 
confirmed  by  the  spectroscope,  which  shows  that 
the  star  is  always  receding  from  us  before  the 
loss  of  light  and  approaching  after  it.  This  result 
is  exactly  what  we  should  expect  on  the  eclipse 
theory,  the  dark  companion  being  so  nearly  of 
the  same  size  as  the  visible  Algol  that  the  joint 
motion  is  similar  to  that  of  two  partners  waltzing 
round  each  other  rather  than  like  the  revolution 
of  a  small  satellite  round  a  large  central  body, 
which  remains  nearly  stationary.  In  other  cases, 
such  as  that  of  /5  Lyrae,  the  intensity  of  the 
light  is  never  constant,  but  undergoes  continual 


ASTRO-PHYSICS  311 

variation,  accompanied  by  complicated  changes  in 
the  spectrum.  A  partial  explanation  is  probably 
to  be  sought  by  imagining  two  luminous  bodies, 
which  revolve  round  each  other,  and  send  to  the 
earth  more  light  when  they  lie  side  by  side  than 
when  one  lies  behind  the  other  and  to  a  certain 
extent  obscures  it. 

It  is  evident  that  the  double  nature  of  such 
systems  can  be  demonstrated  by  variations  in 
luminosity  only  in  the  few  cases  where  the  motion 
is  in  such  a  plane  that  one  of  the  partners  is 
periodically  interposed  between  the  other  and  the 
earth.  Only  about  twenty  Algol  variables  are,  in 
fact,  known.  When  this  eclipse  does  not  happen, 
the  dark  companion  could  never  be  detected  with- 
out the  aid  of  the  spectroscope.  By  continuous 
records  of  the  spectra  of  many  stars,  however, 
periodic  changes  in  the  lines  have  been  observed, 
and  the  times  of  the  orbital  movements  deter- 
mined. Binary  systems  have  been  discovered 
with  periods  varying  from  a  few  hours  to  several 
years.  In  some  cases  the  spectral  changes  merely 
consist  in  periodic  shif tings  of  the  lines.  Here 
we  probably  have  a  luminous  body  and  a  dark 
companion  revolving  round  their  common  centre 
of  gravity.  In  other  cases,  a  periodic  doubling 
of  the  lines  indicates  two  bodies,  both  luminous, 
but  too  near  together  and  too  far  from  us  to  be 


312  PHYSICAL  SCIENCE 

separated  by  the  telescope.  The  number  of  both 
classes  seems  to  be  considerable,  and  our  visible 
universe  must  be  studded  pretty  closely  with  dark 
stars,  the  existence  of  which  is  only  to  be  detected 
in  cases  of  their  combination  with  some  luminous 
companion. 

Passing  from  these  questions  to  the  problems 
of  our  own  planetary  system,  we  find  the  same 
principle  applied  to  the  examination  of  Saturn's 
rings.  These  remarkable  structures,  which  in  the 
telescope  look  like  rings  of  continuous  matter 
encircling  the  planet,  were  long  a  puzzle  to  the 
astronomer.  Theory  indicates  that  such  rotating 
rings  of  continuous  matter,  whether  solid  or  liquid, 
would  be  unstable,  and  would  break  up  under  the 
forces  which  must  necessarily  exist.  The  alter- 
native hypothesis,  that  the  rings  consist  of  a 
swarm  of  tiny  meteorites,  each  revolving  round 
the  planet  in  its  own  separate  orbit,  was  elabor- 
ated mathematically  by  Clerk  Maxwell,  but  no 
confirmation  of  this  view  was  obtained  till  Keeler 
examined  the  rings  with  the  spectroscope.  He 
found  that  the  inner  parts  of  the  rings  revolved 
faster  than  the  outer  parts,  in  accordance  with 
the  requirements  of  the  meteoritic  hypothesis. 
If,  on  the  other  hand,  the  ring  were  solid,  the 
outermost  parts  would  possess  the  highest  velo- 
city, on  the  same  principle  by  which  the  circum- 


ASTRO-PHYSICS  313 

ference  of  a  fly-wheel  moves  faster  than  its  inward 
parts. 

While  the  knowledge  of  sun  and  stars  delivered 
to  us  by  spectrum  analysis  has  been  both  exten- 
sive and  striking,  the  interpretation  of  spectral 
phenomena  has  proved  a  much  more  complicated 
problem  than  was  anticipated  when  Bunsen  and 
Kirchorf's  great  discovery  first  placed  the  new 
method  in  the  hands  of  investigators.  The  lines 
of  the  spectrum,  whether  bright  or  dark,  were 
thought  at  first  to  be  absolutely  fixed  and  constant 
in  position — that  is,  the  modes  of  vibration  of 
the  atoms  from  which  the  light  proceeded  were 
imagined  to  be  unaffected  by  any  external  cir- 
cumstances. This  supposed  simplicity  has  been 
shown  to  be  illusory.  As  we  have  seen,  move- 
ment of  the  source  and  observer,  although  it 
may  not  alter  the  atomic  vibrations,  affects  the 
number  of  them  received  in  any  time,  and  thus 
changes  the  refrangibility  of  the  light  they  emit 
as  it  is  received  by  the  observer.  But  other 
variations,  more  fundamental  in  their  origin, 
are  known.  Laboratory  experiments  have  shown 
that  the  spectral  lines  alter  their  character  with 
changes  in  the  physical  conditions  of  the  experi- 
ments. It  was  thought  that  luminous  gases  evolved 
only  bright,  sharp  lines.  It  is  now  found  that 


314  PHYSICAL  SCIENCE 

the  lines  may  be  broadened  and  softened  by  an 
increase  in  the  pressure  or  the  density  of  the  gas, 
while,  in  some  cases,  a  simultaneous  shift  in  posi- 
tion may  be  produced.  An  intense  magnetic  field 
has  been  shown  by  Zeeman  to  result  in  separation 
of  single  lines  into  two  or  more  components,  in 
this  fulfilling  the  predictions  of  the  electro-mag- 
netic theory  of  light,  which  suggests  that  some 
such  connection  is  probable.  The  spectra  of 
elements  have  long  been  known  to  depend  on  the 
temperature,  the  spectrum  of  the  arc  discharge 
often  being  different  from  that  obtained  by  the 
use  of  a  discontinuous  spark,  while  neither  cor- 
respond to  the  spectrum  of  the  incandescent 
vapour  existing  in  the  flame  of  a  gas-burner. 
More  recent  experiments  have  shown  that  traces 
of  impurities  may  modify  the  spectrum  consider- 
ably, while,  in  some  cases,  the  presence  of  one 
substance  will  completely  mask  the  spectrum  of 
another. 

The  possibilities  introduced  by  all  these  effects 
naturally  complicate  the  interpretation  of  solar 
and  stellar  spectra.  On  the  other  hand,  the  very 
complications  greatly  increase  the  interest  of  the 
luminous  messages,  and  the  investigation  of  the 
connection  between  the  external  conditions  and 
the  nature  of  the  spectra  in  the  physical  laboratory 
opens  an  almost  limitless  field  to  profitable  re- 


ASTRO-PHYSICS  315 

search.  Co-operation  between  the  laboratory  and 
the  observatory  doubtless  will  elucidate  gradually 
the  fascinating  problems  of  the  nature  of  the 
celestial  bodies. 

The  spectra  of  various  substances  differ  widely 
in  complexity.  Some  consist  of  a  few  lines, 
some  of  very  many.  Iron,  for  instance,  emits 
light  of  at  least  two  thousand  different  wave- 
lengths. Of  recent  years  order  has  to  some 
extent  been  introduced  into  our  knowledge  of 
complex  spectra  by  the  discovery  that  fairly 
simple  relations  hold  between  their  wave-lengths 
or  their  frequencies  of  vibration.  Simple  for- 
mulae have  been  devised,  which  in  a  general  way 
express  the  connection  between  the  frequency 
of  one  fundamental  line  and  its  companions, 
somewhat  as  can  be  expressed  the  connection 
between  a  musical  note  and  its  overtones.  In 
many  cases,  however,  it  is  impossible  to  bring 
all  the  lines  of  a  spectrum  into  conformity  with 
such  a  formula.  Two  or  even  three  series  of  lines 
exist,  and  two  or  three  formulae  are  needed  to 
co-ordinate  their  frequencies.  That  such  dis- 
tinctions possess  a  physical  significance  is  shown 
by  recent  experiments  of  Lenard,  who  finds  that 
the  three  series  of  sodium  and  lithium  lines  are 
separated  in  the  flame  of  the  electric  arc,  the  outer 
shell  of  flame  giving  only  the  fundamental  series, 


316  PHYSICAL  SCIENCE 

while,  in  the  physical  conditions  appertaining  to 
the  inner  flame,  the  second  and  third  series  become 
dominant. 

Some  of  the  most  interesting  work  now  in  pro- 
gress relating  to  the  sun  is  founded  on  Professor 
Hale's  method  of  photographing  the  sun  itself 
and  its  prominences  by  the  light  corresponding 
to  one  definite  spectral  line.  Two  of  the  com- 
monest elements  present  in  the  sun  are  hydrogen 
and  calcium,  and  these  elements  are  marked  by 
the  strong  lines  H  and  K  respectively.  The  re- 
sultant photographs,  then,  show  the  distribution 
of  hydrogen  or  calcium  throughout  the  region 
investigated.  The  spectra  of  the  prominences  at 
the  edge  of  the  sun's  disc  consist  of  bright  lines, 
while  some  of  the  dark  absorption  lines  of  the 
light  from  the  surface  of  the  sun  possess  bright 
centres,  like  those  shown  in  Fig.  36,  indica- 
ting the  existence  of  masses  of  luminous  vapour 
lying  above  the  reversing  layer.  These  bright 
central  lines  give  sufficient  light  for  the  purpose 
we  are  now  considering,  and  the  resulting  photo- 
graphs show  the  distribution  of  glowing  clouds 
of  vapour  in  the  higher  regions  of  the  solar 
atmosphere.  Even  the  dark  absorption  lines  are 
only  dark  by  comparison  with  the  brighter  back- 
ground, and  thus  new  photographs  can  be  taken 


ASTRO-PHYSICS  317 

with  the  darker  sides  of  these  reversed  lines. 
The  light  then  used  comes  from  a  deeper  layer 
in  the  solar  atmosphere,  and  as  many  as  three 
calcium  photographs  have  been  taken  in  this  way 
from  a  single  line,  showing  the  distribution  of  cal- 
cium at  three  different  levels  in  the  sun's  envelope. 

The  method  by  which  Professor  Hale  obtains 
these  wonderful  results  consists  in  the  employ- 
ment of  a  spectro-helioscope  possessing  two  slits. 
The  solar  light  is  focussed  into  an  image  by  the 
telescope,  passed  through  one  of  these  slits,  and 
thrown  on  a  prism  or  grating.  The  spectrum  thus 
produced  shows  the  usual  lines,  and  the  second 
slit  is  fixed  so  as  to  coincide  with  the  line  by  the 
light  of  which  the  sun  is  to  be  photographed. 
The  light  coming  through  the  second  slit  is  thus 
monochromatic  light — simple  light  of  the  parti- 
cular kind  desired.  The  first  slit  is  made  to  travel 
slowly  over  the  disc  of  the  sun,  and  the  second 
slit,  by  appropriate  movements,  is  kept  constantly 
in  position  to  allow  the  particular  line  to  fall 
upon  it.  In  this  way  a  complete  picture  of  the 
calcium  or  hydrogen  flames  above  the  surface 
of  the  sun  can  be  obtained. 

One  of  the  striking  features  of  recent  photo- 
graphs taken  by  this  method  consists  in  the  well- 
marked  differences  in  the  distribution  of  hydrogen 
and  calcium.  The  faculse  and  prominences,  which 


318  PHYSICAL  SCIENCE 

stud  the  solar  disc,  contain  floating  clouds  of 
hydrogen,  and  other  clouds  of  calcium,  but  these 
clouds  are  often  separate  from  each  other,  and 
possess  distinctive  forms  which  are  well  shown 
in  Figs.  37  and  38,  and  can  at  once  be  re- 
cognised by  an  accustomed  observer  as  due  to 
hydrogen  or  calcium  respectively.  Prominent 
objects  on  the  sun,  such  as  spots,  often  show 
clearly  only  in  one  of  these  two  kinds  of  light, 
when  they  are  faintly  seen  or  are  quite  invisible 
by  the  other  elemental  ray.  Vast  clouds  of 
calcium  seem  to  arise  from  the  neighbourhood 
of  sun  spots,  obscuring  the  calcium  light  coming 
from  the  regions  below,  while  at  the  same  time 
the  hydrogen  light  from  those  regions  is  able  to 
make  good  its  escape. 

Most  of  the  dark  lines  of  the  solar  spectrum 
are  probably  due  to  elements  known  on  the 
earth,  some  imperfect  coincidences  being  attri- 
buted to  the  difference  in  physical  conditions, 
which,  as  we  now  know,  affect  the  character  of 
the  spectral  lines.  The  bright  lines  of  the  outer 
luminous  layer  or  chromosphere,  and  of  its  at- 
tendant prominences,  were  first  detected  during 
eclipses,  though  with  modern  instruments  they 
can  always  be  seen  at  the  edge  of  the  sun's  disc. 
A  brilliant  unknown  line  in  the  yellow  was  in 


u 


ASTRO-PHYSICS  319 

1868  referred  by  Sir  Norman  Lockyer  to  a 
new  element,  to  which  was  given  the  name  of 
helium.  In  1895  Sir  William  Ramsay  detected 
the  same  spectrum  by  passing  an  electric  spark 
through  the  gases  evolved  from  a  specimen  of 
the  mineral  cleveite,  and  by  this  means  isolated 
the  gas  helium,  thus  showing  that  the  element, 
first  discovered  in  the  sun,  was  present  also 
upon  the  earth.  The  complete  spectrum  of 
helium  contains  two  sets  of  lines,  one  in  the 
yellow  and  one  in  the  green.  In  the  laboratory 
these  two  sets  are  usually  found  together,  though, 
by  manipulating  an  electric  discharge  in  helium, 
separation  may  be  effected.  In  the  light  of  the 
sun,  too,  the  yellow  line  is  sometimes  found  with- 
out the  green.  Other  separations  of  the  same 
kind  between  the  constituents  of  the  spectra  of 
certain  elements  have  also  been  observed,  and  have 
sometimes  suggested  the  idea  of  atomic  dissocia- 
tion. Other  explanations,  however,  seem,  on  the 
whole,  more  probable.  Professor  J.  ].  Thomson 
has  shown  that,  when  an  electric  discharge  passes 
through  rarified  hydrogen,  the  red  line  becomes 
more  intense  near  the  positive  and  the  green  line 
near  the  negative  electrode.  This  observation 
indicates  a  separation  of  hydrogen  molecules  into 
positive  and  negative  parts,  and  is  very  suggestive 
in  relation  to  solar  physics. 


320  PHYSICAL  SCIENCE 

During  total  eclipse,  a  vast  radiance  surrounding 
the  sun,  known  as  the  corona,  springs  into  view. 
Spectroscopic  examination  shows  that  hydrogen, 
helium,  and  calcium,  the  main  constituents  of 
the  chromosphere,  are  absent  in  the  corona.  The 
principal  part  of  the  light  seems  to  be  due  to  a 
brilliant  green  line,  not  produced  by  any  terrestrial 
substance.  The  hypothetical  element  emitting  this 
light  has  been  named  coronium. 

Although  recent  research  has  not  yet  led  to  a 
completely  satisfactory  conception  of  the  general 
condition  of  the  sun  as  a  physical  system,  sub- 
stantial progress  in  knowledge  has  nevertheless 
been  made.  The  gigantic  output  of  heat  would 
be  impossible  for  any  solid  globe,  even  if  sur- 
rounded by  a  gaseous  envelope.  The  external 
shell  would  cool  too  rapidly,  unless  a  process 
of  convection  replaced  the  cooling  gases  on  the 
surface  by  hotter  ones  from  below.  The  tem- 
perature of  the  sun  is  above  the  critical  points 
of  most,  at  any  rate,  of  known  substances,  and 
thus,  although  the  pressures  may  be  very  high, 
liquids  or  solids  are  probably  non-existent,  except 
perhaps  as  clouds  in  the  upper  regions  of  the 
atmosphere.  The  best  estimates  of  the  tempera- 
ture of  the  radiating  part  of  the  sun,  based  on 
the  amount  of  solar  heat  received  by  the  earth, 


ASTRO-PHYSICS  321 

corrected  for  absorption,  agree  in  indicating  a 
temperature  of  about  6000°  C. 

A  fairly  general  consensus  of  opinion  had  been 
reached  to  the  effect  that  the  source  of  the  energy 
required  for  the  sun's  continual  output  of  heat 
was  to  be  sought  in  the  mutual  gravitating  con- 
densation of  his  parts.  A  mass  of  gravitating  gas 
may  become  actually  hotter  by  radiation.  As  it 
loses  heat,  its  parts  approach,  and  the  whole  mass 
contracts.  Two  bodies  attracting  each  other  will, 
by  their  collision,  set  free  energy  which  appears 
as  heat,  and  the  mutual  approach  of  the  gravitating 
parts  is  an  effect  of  the  same  kind.  The  heat 
thus  developed  may  be  more  than  enough  to 
compensate  for  that  lost  by  radiation.  This 
reasoning  was  applied  to  the  sun,  and  estimates  of 
the  sun's  life  as  a  useful  radiating  system  were 
made  by  Lord  Kelvin  and  others.  But  the  past 
history  of  the  sun  was,  on  these  calculations,  far 
too  short  to  admit  of  the  periods  required  by  the 
geologist  and  the  biologist  for  the  formation  of  the 
earth's  crust  and  the  evolution  of  species  thereon. 

The  phenomena  of  radio-activity  have,  how- 
ever, thrown  new  light  on  this  problem.  If  but 
two  or  three  parts  in  a  million  of  the  sun's  mass 
consist  of  radium,  Mr.  W.  E.  Wilson  has  shown 
that  the  present  rate  of  heat  emission  would  be 
maintained.  Thus  a  very  small  quantity  of 

x 


322  PHYSICAL  SCIENCE 

radio  -  active  material  would  appreciably  retard 
the  loss  of  heat,  and  greatly  prolong  the  possible 
life  of  the  sun.  The  spectrum  of  radium  does 
not  show  in  the  sun's  light,  but,  as  laboratory 
experiments  indicate,  a  large  proportion  of  radium 
would  be  necessary  to  make  visible  its  char- 
acteristic lines.  On  the  other  hand,  the  pre- 
valence of  helium  suggests  the  occurrence  of  radio- 
active processes,  during  which,  as  we  know,  helium 
may  be  formed. 

A  similar  lengthening  of  the  probable  age  of 
the  earth  is  also  indicated  by  the  same  course 
of  argument.  The  temperature  of  the  earth  rises 
as  we  pass  underground,  and,  from  the  present 
temperature  gradient,  Lord  Kelvin  had  calculated 
that  about  one  hundred  million  years  ago  the 
earth  was  a  molten  mass.  Although  from  the 
nature  of  the  assumptions  made  in  this  calcula- 
tion, little  weight  could  be  attached  to  the  exact 
result  obtained,  the  estimated  age  of  the  earth,  as 
the  home  of  organic  life,  was  again  too  short  for 
the  requirements  of  geology  and  biology.  But  it 
is  now  known  that  radio-active  matter  in  small 
quantities  is  very  widely  distributed  throughout 
the  earth  and  its  atmosphere.  Clay,  for  instance, 
yields  a  radio-active  emanation  in  appreciable 
quantities,  and  Professor  Rutherford  has  calcu- 
lated that,  if  all  the  substance  of  the  earth  were 


ASTRO-PHYSICS  32 

as  active  as  clay,  the  present  distribution  of  tem- 
perature might  be  maintained  by  this  cause  alone. 
Such  a  result  shows,  at  all  events,  that  the  ob- 
served temperature  gradient  is  not  a  safe  guide 
when  used  as  the  sole  means  of  estimating  the 
age  of  the  habitable  globe. 

The  great  advance  in  knowledge,  recently 
gained  by  the  study  of  the  conduction  of  elec- 
tricity through  gases  and  the  phenomena  of 
radio-activity,  cannot  fail  to  exert  a  powerful 
influence  on  the  future  of  astro-physics,  and,  in 
particular,  on  our  conceptions  of  the  nature  of 
solar  processes.  The  leak  of  electricity  from  hot 
bodies,  studied  in  the  physical  laboratory,  shows 
that  corpuscles  or  electrons  must  be  emitted  in 
enormous  quantities  by  the  substance  of  the  sun 
and  hot  stars.  The  likelihood  of  the  presence 
of  radio-active  matter,  too,  and  of  the  ejection  of 
other  corpuscles,  with  the  transcendent  velocities 
impressed  on  them  by  a  radio-active  origin,  must 
not  be  forgotten.  Although  the  corpuscles,  before 
they  reached  the  surface  of  the  earth,  would  be 
absorbed  by  its  atmosphere — equivalent  as  that 
atmosphere  is  to  a  thickness  of  thirty  inches  of 
mercury — they  might  produce  striking  phenomena 
in  the  regions  of  the  upper  air.  Perhaps  on  these 
lines  is  to  be  explained  the  appearance  of  the 


324  PHYSICAL  SCIENCE 

Aurora  Borealis  and  kindred  manifestations,  while 
the  luminosity  of  the  solar  corona  may  well  have 
an  electric  origin.  The  application  of  the  new 
discoveries  of  the  laboratory  to  the  problems  of 
the  heavens  has  hardly  yet  begun;  but  the  region 
of  investigation  thus  opened  up  must  prove  of 
almost  illimitable  extent,  and  the  coming  advance 
in  its  exploration  will  be  followed  with  absorbing 
interest  in  the  ensuing  years. 

The  spectroscopic  study  of  the  stars  has  led  to 
their  classification  into  four  main  groups,  clearly 
distinguished  by  Father  Secchi.  The  first  type 
consists  of  white  stars  showing  the  effects  of 
strong  helium  and  hydrogen  absorption.  The 
second  give  spectra  crossed,  like  that  of  our  sun, 
by  innumerable  fine  metallic  lines.  The  third  type 
includes  red  stars  with  banded  spectra,  the  bands 
being  diffuse  on  their  redder  sides.  In  the  fourth 
group  are  placed  faint  dark  red  stars,  possessing 
wide  bands  in  their  spectra  attributed  to  carbon. 
The  addition  of  a  fifth  class,  including  stars  with 
bright  lines  in  their  spectra,  has  been  suggested  by 
Pickering.  Many  subdivisions  of  these  types  have 
been  recognised,  and  the  variety  and  complexity 
of  stellar  natures  seem  endless. 

It  is  impossible  to  resist  the  conclusion,  how- 
ever, that  in  this  classification  we  have  traced 


ASTRO-PHYSICS  325 

the  main  outlines  of  the  normal  course  of  stellar 
evolution.  In  their  youth,  the  suns  seem  to  be 
surrounded  with  atmospheres  principally  consisting 
of  helium  and  hydrogen.  In  their  magnificent 
and  turbulent  prime,  they  are  swathed  in  glowing 
robes  of  metallic  vapour,  still  covered  with  the 
gauzy  veil  of  helium  and  hydrogen.  As  they 
decline  in  vigour,  their  light  grows  redder,  like 
that  of  a  cooling  iron  bar.  Of  their  ultimate  con- 
dition, the  same  analogy,  and  the  inferred  existence 
of  dark  companions,  give  us  some  suggestion. 
Evidence,  too,  indicating  the  occasional  possibility 
of  a  stellar  resurrection  is  not  withheld  from  us. 

Many  difficulties  of  interpretation  still  perplex 
the  astronomer  ;  observations  accumulate  and  await 
explanation  in  increasing  number.  Nevertheless,  it 
is  probable  that  in  some  such  life-history  as  this, 
we  have  already  correctly  formulated  the  true  course 
of  evolution  of  the  majority  of  the  stars. 

The  appearance  of  temporary  stars  is  a  pheno- 
menon which  has  been  observed  repeatedly  in 
historical  times.  Hipparchus,  Tycho  Brahe,  and 
Kepler,  for  instance,  have  recorded  such  manifes- 
tations. But  the  first  case  critically  examined  by 
modern  photographic  methods  was  that  of  Nova 
Aurigae,  a  star  discovered  in  February  1892,  the 
origin  and  growth  of  which  were  traced  by  subse- 


326  PHYSICAL  SCIENCE 

quent  examinations  of  photographs  taken  in  the 
previous  December  and  January,  and  preserved  as 
part  of  the  systematic  photographic  log-book  of 
the  heavens  now  kept  by  astronomers.  For  three 
months  the  star's  brightness  lasted  and  then  rapidly 
it  decreased,  till  at  the  end  of  April  the  Nova  was 
barely  visible  in  the  great  refracting  telescope  of 
the  Lick  Observatory.  Soon  afterwards,  however, 
a  faint  nebula  appeared  in  its  place,  with  a  quite 
different  kind  of  spectrum. 

More  completely  studied  were  the  striking 
phenomena  of  the  second  Nova  Persei,  first 
sighted  at  Edinburgh  in  February  1901.  Its 
rise  and  decline  were  followed  in  many  places, 
particularly  by  Father  Sidgreaves  at  Stonyhurst, 
and  by  Professor  Campbell  at  the  Lick  Observa- 
tory. It  attained  its  maximum  brightness  about  a 
day  and  a  half  after  its  detection,  and  then  grew 
fainter  in  a  fluctuating  manner  for  about  ten  days. 
Finally,  a  nebula  was  seen  to  develop,  which  in- 
creased in  visible  dimensions  at  a  prodigious  rate — 
so  fast,  indeed,  that  the  most  probable  explanation 
supposes  that  the  nebula  was  pre-existent  but 
non-luminous,  and  was  made  visible  by  the  flood 
of  light  released  by  the  star.  That  light  was  re- 
flected as  it  spread  outwards  from  the  centre  in 
ever-widening  spheres,  and  illuminated  the  scattered 
wisps  of  attenuated  matter  it  encountered  on  its  way 


ASTRO-PHYSICS  327 

through  space.  Calculating  from  this  assumption, 
it  is  obviously  possible  to  deduce  the  distance  of 
the  star,  which  proves  to  be  such  that  light  would 
take  about  three  hundred  years  to  reach  our  eyes. 
It  would  follow  that  the  phenomena  we  studied  in 
the  last  days  of  Queen  Victoria  represented  changes 
that  were  occurring  in  the  depths  of  space  while 
Queen  Elizabeth  occupied  the  throne  of  England. 
When  examined  spectroscopically,  the  light  of 
all  the  temporary  stars  yet  investigated  shows  one 
remarkable  property.  Bright  lines,  displaced  to- 
wards the  red,  are  accompanied  by  dark  lines 
of  similar  origin  displaced  towards  the  violet. 
Doppler's  principle  would  indicate  that  the  source 
of  these  double  lines  was  a  double  star,  the  bright 
lines  coming  from  a  gaseous  system  emitting  a 
line  spectrum,  and  the  dark  lines  from  a  partner 
star  in  which  absorption  was  predominant.  But 
the  difficulties  of  such  a  view  seem  insuperable. 
The  requisite  velocities  are  of  the  order  of  some 
hundreds  of  miles  a  second,  and  no  sign  of 
periodicity  or  even  diminution  appears  in  their 
values.  At  one  time  it  was  thought  that  the  tem- 
porary blaze  of  light  might  be  due  to  the  shock  of 
collision  of  two  stars  meeting  in  space  ;  but  the 
doubling  of  the  spectral  lines  indicates  a  common 
constitution  unlikely  invariably  to  be  possessed  by 
disconnected  systems  flying  through  the  aether 


328  PHYSICAL  SCIENCE 

from  distant  sources.  On  the  other  hand,  the 
opposite  velocities,  constant  in  amount,  show  that 
the  two  stars  cannot  be  two  members  of  the  same 
group,  colliding  with  each  other  as  an  effect  of 
ill-directed  mutual  gravitation,  which  would  lead 
to  a  decrease  in  velocity  as  the  stars,  after  collision, 
receded  from  each  other.  The  theory  of  collision 
has  perforce  been  abandoned.  No  satisfactory 
hypothesis  has  yet  been  proposed  in  its  place. 
Perhaps  the  one  least  open  to  objection  is  that 
which  regards  the  luminosity  as  due  to  the  passage 
of  a  star,  possibly  a  dark  one,  through  the  scattered 
matter  constituting  a  nebula,  in  much  the  same  way 
as  a  shooting  star  shines  only  during  its  transit 
through  the  earth's  atmosphere. 

Many  years  ago  Clerk  Maxwell  showed  theo- 
retically that  a  stream  of  light,  incident  on  a 
body,  should  produce  a  pressure  in  the  direc- 
tion of  the  advancing  rays.  Maxwell  deduced 
the  effect  from  the  electro-magnetic  theory  of 
light,  but  it  has  since  been  shown  by  Larmor 
to  be  necessary  on  almost  any  wave  theory. 
The  undulating  medium  possesses  energy,  and, 
therefore,  momentum.  An  absorbing  body  is 
gaining  momentum,  and  therefore  experiences  a 
pressure  in  the  direction  of  the  incident  beam. 
A  reflecting  body  reflects  the  same  momentum 


ASTRO-PHYSICS  329 

back  again,  and  therefore  is  acted  on  by  a 
double  pressure.  This  result  has  recently  been 
confirmed  experimentally  by  Professor  Lebedef, 
of  Moscow.  The  difficulties  to  be  overcome 
are  best  appreciated  by  the  statement  that  when 
bright  sunlight  falls  on  a  reflecting  surface,  the 
pressure  to  be  detected  amounts  to  less  than  a 
milligramme  per  square  metre.  For  an  absorbing 
surface  such  as  lamp  black,  the  pressure  is  half 
as  great  as  for  a  reflector,  and  it  is  the  difference 
between  these  two  effects  that  M.  Lebedef  has 
detected,  the  results  of  unequal  heating  and  of 
molecular  recoil  being  successfully  eliminated. 
By  another  method  the  same  pressure  has  still 
more  recently  been  demonstrated  by  Nichols  and 
Hull. 

Owing  to  this  pressure,  two  bodies  radiating 
towards  each  other  will  experience  a  mutual 
repulsion,  which,  for  small  particles,  may  over- 
come the  gravitational  attraction.  Even  the 
attraction  of  the  sun  on  a  body  may  be  neutra- 
lised if  the  body  is  of  minute  size,  for  the  radia- 
tion effect  depends  on  the  area  of  surface, 
while  the  weight  depends  on  the  volume.  As 
the  size  is  diminished,  the  area  decreases  less 
rapidly  than  the  volume,  and,  for  microscopic 
particles  less  than  o.oooi  millimetre  in  diameter, 
the  radiative  repulsion  of  the  sun  becomes  greater 


330  PHYSICAL  SCIENCE 

than  the  gravitational  attraction.  An  interesting 
application  of  this  principle  has  explained  the 
curious  phenomena  of  comets'  tails,  which  have 
long  puzzled  the  ingenuity  of  astronomers.  If, 
as  is  probable,  a  comet  consists  of  a  collection  of 
meteorites,  varying  in  size  from  small  worlds  to 
microscopic  particles,  on  approaching  the  sun  the 
large  masses  will  follow  the  parabolic  path  ABC 
(Fig.  39),  indicated  by  the  ordinary  gravitational 
theory.  Particles  of  the  particular  size  at  which 
the  radiative  force  just  balances  that  due  to  gravity 
will  pursue  a  path,  ADE,  in  an  undeviated  course, 
for  both  the  forces  vary  inversely  as  the  square 
of  the  distance,  and  will  thus  balance  each  other 
at  all  distances.  Particles  intermediate  in  size 
will  follow  intermediate  paths,  AF,  AG,  AH, 
&c.  As  the  comet  swings  round  the  sun,  the 
tail  becomes  expanded  into  the  fan-like  form 
commonly  observed.  The  head  of  the  comet 
goes  on  its  way  into  the  depths  of  space,  having 
lost  some  of  the  smaller  constituents  of  its  tail, 
which  are  scattered  throughout  interplanetary 
regions. 

Not  only  does  the  radiation  from  the  sun 
cause  a  repulsion  of  small  objects,  but  their 
radiation  to  each  other  will,  as  Professor  Poynt- 
ing  has  just  recently  shown,  lead  to  a  mutual 


FIG.  39.— DIAGRAM  TO  EXPLAIN  THE  PHENOMENA 
OF  COMETS'  TAILS 


To  face  page  330 


ASTRO-PHYSICS  331 

repulsion  when  the  bodies  are  placed  in  a  region 
of  space  where  the  effective  temperature  is 
lower  than  their  own.  Two  meteorites  at  ordi- 
nary temperatures,  say  at  300°  on  the  absolute 
scale,  will  in  cold  space  repel  each  other  with 
a  force  equal  to  their  mutual  gravitative  attrac- 
tion when  their  radii  are  about  3.4  centimetres, 
and,  in  the  case  of  smaller  bodies,  the  repulsion 
will  overcome  the  gravitative  effect.  In  this  case, 
when  the  gravitational  force  is  that  between  bodies 
of  small  mass,  instead  of  that  between  some  small 
body  and  the  gigantic  sun,  a  resultant  repulsion 
is  reached  at  much  larger  dimensions  than  those 
of  the  case  formerly  considered.  It  is  evident 
that  a  swarm  of  meteorites  of  the  right  size 
might  continue  to  revolve  round  a  planet  or 
sun  without  mutual  forces  and  independently 
of  each  other.  It  is  possible  that  this  result 
has  some  bearing  on  the  problem  of  Saturn's 
rings. 

A  curious  conclusion  may  be  drawn  from  the 
theory  of  the  radiation  -  force  between  small 
bodies.  Unless  the  temperatures  are  the  same, 
the  force  on  one  need  not  necessarily  be  equal 
to  the  force  on  the  other:  action  and  reaction 
it  seems  are  not  equal  and  opposite.  The  in- 
consistency is,  of  course,  prevented  if  we  re- 
member that  the  momentum  of  the  aether  must 


332  PHYSICAL  SCIENCE 

also  be  taken  into  account.  In  reality  each 
body  is  emitting  a  stream  of  momentum  which 
exists  for  a  while  in  the  medium.  In  the  inter- 
action between  that  medium  and  either  body, 
Newton's  laws  still  hold.  This  is  an  example 
of  the  important  part  now  played  by  the  aether 
in  physical  conceptions.  Its  existence  may  be 
hypothetical,  but  its  properties,  hypothetical  or 
not,  are  required  to  correlate  the  phenomena  of 
the  universe,  and  enter  into  the  calculations  in 
which  the  results  of  observation  are  expressed. 
Constantly  the  energy  and  momentum  of  the 
aether  seem  to  be  exchanged  with  those  of 
ordinary  matter,  and  to  be  just  as  much  phy- 
sical realities. 

If  we  neglect  the  effect  of  the  aether,  there 
is  no  reason,  in  the  case  considered,  why  action 
and  reaction  should  be  equal  and  opposite.  It 
is  even  possible  to  imagine  the  gravitation  pull 
and  the  radiation  push  so  adjusted  that  the 
accelerations  become  equal  but  in  the  same 
direction.  The  hotter  body  will  then  chase  the 
colder  body  through  space  with  constantly  in- 
creasing velocity.  A  limit  will,  however,  eventu- 
ally be  reached,  for,  owing  to  the  Doppler 
principle,  the  waves  in  front  of  a  moving  body 
are  crowded  up,  and  those  behind  it  lengthened 
out.  The  radiation-pressure  in  front  is  thus 


ASTRO-PHYSICS  333 

increased,  and  that  behind  diminished,  so  that 
the  net  result  is  a  retardation  which  tends  to 
check  the  motion.  In  the  case  of  meteorites 
small  but  yet  large  enough  for  the  gravitative 
pull  to  be  predominant,  which  are  revolving  round 
large  bodies  in  orbits  with  high  speeds,  this  retar- 
dation becomes  important,  and  will  eventually 
cause  the  meteorites  to  gravitate  towards  the 
centre.  In  this  way  it  is  possible  that  the  sun 
may  clear  the  neighbouring  space  of  meteoritic 
dust,  which  would  otherwise  move  round  him  in 
permanent  orbits  ;  and  the  earth  would  draw 
back  to  herself  any  particles  shot  out  by  vol- 
canic eruptions,  such  as  that  of  Krakatoa,  when 
the  velocities  impressed  may  have  been  great 
enough  to  carry  them  beyond  the  atmosphere, 
and  in  the  right  direction  to  set  them  moving 
as  satellites. 

The  theory  of  radiation  also  enables  us  to 
solve  many  other  interesting  problems  connected 
with  the  solar  system.  By  means  of  a  thermo- 
dynamic  proof  it  has  been  shown  that  the  total 
radiation  from  a  source  should  vary  as  the  fourth 
power  of  the  absolute  temperature  T,  that  is, 
as  r4.  By  experimental  investigation  it  is  pos- 
sible to  establish  a  numerical  relation,  and, 
if  JR  be  the  energy  radiated  per  square  centi- 
metre per  second  by  a  full  radiator  such  as 


334  PHYSICAL  SCIENCE 

lamp  black,  the  constant  k  in  the  theoretical 
equation 

R  =  kT* 

has  been  found  by  Kurlbaum  to  be  about  5.32 
io-5  erg.* 

Now  we  can  calculate  the  total  energy  radiated 
from  the  sun  per  second  by  measuring  the 
amount  received  at  the  surface  of  the  earth, 
and  estimating  the  amount  lost  by  reflection 
and  absorption  by  the  atmosphere.  These  con- 
siderations lead  directly  to  the  effective  tempera- 
ture of  the  sun,  which  is  thus  estimated  to  be 
from  6200°  to  7000°  absolute.  Professor  Poynt- 
ing  prefers  the  lower  value,  which  means  about 
6000°  C. 

A  small  body,  isolated  in  space,  will,  when 
a  steady  state  is  reached,  radiate  as  much  heat 
as  it  absorbs.  If  it  be  shielded  from  the  sun,  it 
will  attain  a  temperature  which  may  be  con- 
sidered to  be  the  effective  temperature  of  space. 
From  estimates  of  the  amount  of  heat  received 
from  the  stars,  as  compared  with  that  received 
from  the  sun,  Poynting  calculates  the  effective 
temperature  of  space  to  be  10°  absolute,  or 
263°  C.  below  the  freezing-point  of  water. 

*  The  erg  is  the  French  unit  of  work  or  energy.  About  an  erg 
of  work  is  done  when  the  thousandth  part  of  a  gramme  is  raised 
through  one  centimetre. 


ASTRO-PHYSICS  335 

Similar  principles  give  a  basis  for  a  deter- 
mination of  the  temperatures  of  planets  at  any 
given  distance  from  the  sun.  Assuming  that  all 
the  heat  absorbed  is  eventually  radiated  out 
again,  and  that  about  one-tenth  of  the  incident 
heat  is  reflected,  and  making  certain  simplify- 
ing assumptions,  the  mean  temperature  of  the 
surface  of  the  earth  is  calculated  as  290°  ab- 
solute, or  17°  C.  The  average  temperature 
of  the  earth's  surface  is  known  to  be  about 
60°  F.,  or  1 6°  C.  The  calculation  is  made 
on  the  assumption  that  the  effective  temperature 
of  the  sun  is  6200°  absolute,  and  its  concord- 
ance with  observation  is  the  ground  given  by 
Poynting  for  prefering  that  value  for  the  solar 
temperature. 

This  success  in  calculating  the  effective  tem- 
perature of  the  earth  lends  weight  to  the  values 
given  by  the  same  method  for  the  temperatures 
of  the  other  planets.  Mercury  and  Venus,  with 
orbits  inside  that  of  the  earth,  possess  tempera- 
tures of  194°  and  69°  C.  respectively,  while  the 
outer  planets,  Mars  and  Neptune,  fall  as  low  as 
—  38°  and  —221°.  If  there  are,  indeed,  inhabi- 
tants on  Mars,  it  seems  that,  according  to  terres- 
trial ideas,  they  must  lead  a  very  chilly  existence. 

When  any  branch  of  learning  first  finds  itself 


336  PHYSICAL  SCIENCE 

in  a  position  to  use  the  methods  and  accumu- 
lated experience  of  another  science,  a  period 
of  striking  discoveries  may  confidently  be  antici- 
pated. Thus  it  was  that  Newton  applied  to  the 
phenomena  of  the  heavens  the  mechanical  know- 
ledge of  previous  ages,  and  the  law  of  gravity 
revealed  the  harmony  of  the  spheres.  When  it 
was  found  that  the  generalisations  of  thermo- 
dynamics and  of  electrical  science  could  be 
used  in  chemical  problems,  a  new  world  opened 
before  the  investigator.  So  it  is  with  the  trans- 
fer of  physical  methods  and  data  to  the  prob- 
lems of  astro-physics.  The  first-fruits  of  this 
harvest  of  knowledge  have  already  proved  of 
momentous  import,  and  in  the  combination  of 
physics  and  astronomy  the  present  labourers 
and  those  that  come  after  them  may  hope  to 
find  one  of  the  most  fertile  unions  in  the  whole 
realm  of  Natural  Philosophy. 


A  List  of  Books 

in  which  may  be  seen  further  particulars  of  the 
subjects  discussed  in  this  volume 


CHAPTER  I.— The   Philosophical   Basis   of   Physical 
Science. 

Die  Mechanik  in  ihre  Entwickelung,  by  E.  MACH  ; 
an  English  translation  by  T.  J.  M'CORMACK  has 
appeared. 

CHAPTER  II. — The  Liquefaction  of  Gases. 

Article  on  Liquid  Gases,  by  Sir  J.  DEWAR,  in  the 
Supplement  to  the  "  Encyclopaedia  Britannica." 

CHAPTER  III.— Fusion  and  Solidification,  and 

CHAPTER  IV.— The  Problems  of  Solution. 

The  Theory  of  Solution,  by  W.  C.  D.  WHETHAM. 

CHAPTER  V. — The  Conduction  of  Electricity  through 
Gases. 

Conduction  of  Electricity  through  Gases,  by  J.  J. 
THOMSON. 

CHAPTER  VI.— Radio -Activity. 

Radio-Activity,  by  E.  RUTHERFORD. 

CHAPTER  VII. — Atoms  and  ^Ether. 

Electricity  and  Matter,  by  J.  J.  THOMSON. 
^Ether  and  Matter,  by  J.  LARMOR. 

CHAPTER  VIII. — Astro -Physics. 

Problems  in  Astro-Physics,  by  Miss  A.  CLERKE. 


LONDON  :   JOHN  MURRAY,  ALBEMARLE  STREET 

Y 


INDEX 


a  RAYS,  203,  204,  208,  210,  230, 

235)  236 

Aberration  of  light,  268 
Absorption,  181 
Actinium,  200 
Adams,  295 
jEther,  9,  42,  192,  244,  264  et 

seq.,  276,  278,  289,  291,  332 
Ethereal  strain,  278,  279,  280, 

288,  289,  291 
Aitken,  156 
Algol,  310,  311 
Aluminium,  89 
Andrews,48;  another  Andrews, 

104 

Antimony,  91,  105 
Argon,  62,  64 
Arrhenius,  2,  123,  146,  189 
Astro-physics,  9,  189,   293  et 

seq. 
Atomic     disintegration,     227, 

228,   234,  236,   240,  284  et 

seq. 
Atomic  structure,  180  el  seq., 

253>  256,  258  et  seq.,  264, 

282,  284  et  seq. 
Atomic  theory,  2,  3,  26,   119, 

122,  126,  244  et  seq. 
Atoms  and  aether,  244  et  seq. 
Aurora  borealis,  189,  324 


(3  RAYS,  203, 204,  210,  235,  266 

Bacon,  Lord,  13 

Barium,       connection       with 

radium,  200 
Becquerel,  199,  222 
Beilby,  G.  T.,  105 
Bemmelen,  Van,  137 
Be'mont,  200 
Bolometer,  302 
Boscovich,  263 
Bronzes,  95 

Buchanan,  ].  Y.,  84,  85 
Bunsen,  296,  297 


CAILLETET,  50,  51 
Calcium  light  from  sun,  316 
Campbell,  326 
Cathode  rays,  162,  172  et  seq., 

255 

Cause  and  effect,  29 
Cavendish,  63 
Chemical  combination,  91, 121, 

134,  246,  253 
Clausius,  66 
Cloud  formation,  1 56 
Coagulation,    138,    140,    143, 

221 

Colloids,  135  et  seq.,  141,  142 
Comets'  tails,  330 


338 


INDEX 


339 


Condensation  nuclei,  156 
Conduction       of      electricity 

through  gases,  3,  8,  148  et 

seq. 

Cooke,  217 
Copper,  86,  91,  95 
Corona,  320 
Coronium,  320 
Corpuscle,  size  of,  281 
Corpuscles,  3,  8,  42,  156,  180 

et  seq.,  255,  258,  262,  270, 

279 
Corpuscular  theory    of  light, 

265 
Crookes,   Sir  Wm.,  162,  163, 

1 8 1,  207,  222,  246 
Cryohydrates,  82 
Curie,    M.  et   Mme.,  3,    198, 

199,    200,     203,     210,    211, 

234 


DALTON,  246,  253 
Darwin,  289 

Democritus,  181,  262,  263 
Dewar,  Sir  J.,   8,  58,  68,  69, 

72,  74,  207,211,  234 
Diffusion,  136,  155,244,249 
Dissociation,  ionic,    123,  124, 

131,  133.  139 
Doppler's  principle,  303^  seq., 

327,  332 

Double  stars,  310 
Dust  nuclei,  1 56 


EARTH,  AGE  OF,  322 
Elasticity  of  the  aether,  268 
Electric  charge,  nature  of,  276, 
278 


Electric  deflection,    173,   177, 

203 

Electric  inertia,  184,  185 
Electrolysis,    2,    125    et  seq., 

109 
Electromagnetic  waves,  9,  269 

et  seq.,  274 
Electrons,  3,  8,  42,  183,  262, 

270,  279,  288 

Electroscopes     and     electro- 
meters, 149 
Elster,  187 
Emanations,  radio-active,  208, 

211,  215,  230,  238 
Energetics,  4,  13,  26,  38,  93, 

115,  119,  122 
Entropy,  40 

Equilibrium,  I,  4,  7,  246 
Eutectic  alloy,  88,  99,  91 
Evaporation,  46,  49,  59 
Eve,  A.  S.,  205 
Evolution  of  matter,  9,  36, 240, 

242,  288 


FARADAY,  2,  48,  125,  126, 
138,  167,  192,  193,  195,  196, 
270 

Fleming,  74 

Fluorescence  and  phosphor- 
escence, 162,  198,  207 

Force,  25 

Foucault,  296,  297 

Frankland,  64 

Fraunhofer,  295 

Freezing-point  curves,  fig.  6, 
p.  87  ;  fig.  7,  P-  90 ;  fig-  8, 
p.  91  ;  fig.  9,  p.  94  5  fig-  io> 
p.  96  ;  fig.  1 8,  p.  103 

Fresnel,  266 


340 


INDEX 


Fusion    and    solidification,  8, 
45,78 


y  RAYS,  203,  205,  210,  235 

Galileo,  27,  31 

Gases,  conduction  of  electricity 

through,  3,  8,  148  et  seq. 
Gay  Lussac,  53 
Geitel,  187 
Gelation,  137 
Gibbs,  Willard,  I,  6,  93,  115, 

146 

Giesel,  213 
Gold,  89 

Graham,  135,  146 
Grain  theory  of  the  aether,  289 
Grating,  300 

Gravitation,  nature  of,  276 
Guthrie,  82 
Gyroscope,  277 


HALE,  G.  E.,  307,  316,  317 
Hardy,  W.  B.,  137,  141,  220 
Heaviside,  281 
Helium,  64,  69,233,  319 
Helmholtz,  Von,  5, 66, 115,1 26, 

195,  196,  263 
Hertz,  269 
Heycock,  C.  T.,  85,  95,   9$, 

101 

Hipparchus,  325 
Hittorf,  2,  126,  129 
Huggins,  Sir  Wm.  and  Lady, 

207,  299 
Hull,  329 
Huygens,  25,  267 
Hydrogen  light  from  sun,  316 
Hypnotism,  36 


ICE,  STRUCTURE  OF,  82 

Induction  and  deduction,  33 
Internal  work  of  gases,  51,  52, 

53,54 

Introduction,  i 
Ionic  charge,   125,    126,    159, 

172,  174,  175 
Ionic  theory,  2,  8,   123,    125, 

131,  139,  150,  190 
Ionic   velocities,    127  et  seq., 

153 
lonization  of  gases,  148,  150, 

153,  157,  164,206,  274 
Iron,  101,  315 


JEANS,  J.  H.,  288 
Joule,  53,  54 


KAHLENBERG,  135 
Kaufmann,  184,  185,  281 
Keeler,  312 
Kelvin,   Lord,   4,   53,   54,  66, 

248,251,263,277,321 
Kepler,  32,  325 
Kirchoff,  296,  297 
Kohlrausch,  2,  126,  129 


LANGEVIN,  153 

Langley,  S.  P.,  302 

Laplace,  19 

Larmor,  J.,  3,  6,  9,  115,  184, 

251,262,277,279,  284,  289, 

328 
Laws   of  Nature,   28,  30,  32, 

35,37 

Leak  of  electricity  from   hot 
surfaces,  187  et  seq^  323 


INDEX 


Lebedef,  329 

Le  Chatelier,  91,  104 

Lenard,  180,  315 

Leverrier,  295 

Linder,  138 

Lines  offeree,  167  et  seq.,  193, 

270  et  seq. 

Liquefaction  of  gases,  8,  45 
Lockyer,  Sir  N.,  64,  319 
Lodge,  Sir  O.  J.,  127,  128,  285 
Lorentz,  3,  262,  279,  284 
Low     temperature     research, 

73 

Lucretius,  262,  291 
Lyrae,  £,310 


MACH,  7,  18,  19 

M'Clung,  R.  K.,  205,  274 

M'Lennan,  217 

Magnetic  deflection,  173,  177, 

203 

Magnets,  equilibrium  of  float- 
ing, 256,  287 
Mars,    temperature     of    the 

planet,  335 
Mass,  22,  26,  38,  184  et  seq., 

278,  279 
Mass  of  ions,  corpuscles,  and 

electrons,   174  et  seq.,    184, 

185 

Masson,  Orme,  128 
Mathematics,  33 
Matter,  22,  26,  40,  180  et  seq., 

278,  279,  280,  288,  290 
Maxwell,    Clerk,    5,  93,   169, 

192,  195,  269,  312,  328 
Mayer,  256,  257 
Mechanics,  7,  17 
Mendeleeff,  254,  261 


Mercury,  temperature  of  the 

planet,  335 

Metallic  conduction,  190  et  seq. 
Metals,   structure  of,   82,   98, 

104,  106 
Metaphysics,   12,  14,  22,  186, 

251 
Microscopic  study  of  metals, 

8,  79,  83,  98,  104 
Molecular    structure,    dimen- 
sions of,  247  et  seq. 
Molecular  theory,  2,  3,  26,  119, 

122,  244  et  seq. 
Momentum,  conservation    of, 

40 


NEBULAE,  298 

Neptune,  temperature  of  the 
planet,  335 

Nernst,  134 

Neville,  F.  PL,  85,  95,  96,  101 

Newton,  Sir  Isaac,  Frontis- 
piece, 13,  25,  26,  32,  263, 
264,  266,  295 

Nicholls,  329 

Novae,  325  et  seq. 


OHM'S  LAW,  132,  151 

Olszewski,  58 

Osmond,  104 

Osmotic  pressure,  112  et  seq. 

123 
Ostwald,  121 


FENDER,  169 
Periodic  law,  254,  261 
Perrin,  172 


342 


INDEX 


Persei,  ft,  310 

Pfeffer,  109,  no,  112,  113 

Phases,  7,  101 

Philosophical  basis  of  physical 
science,  6,  1 1  et  seq. 

Phosphorescence  and  fluores- 
cence, 162,  198,  207 

Phosphorescence  at  low  tem- 
peratures, 75 

Photography  applied  to  astro- 
physics, 299,  316 

Physiology,  16,  108,  137,  143, 
220 

Pickering,  324 

Pictet,  50,  51 

Picton,  138 

Pitch-blende,  200,  240 

Planck,  123 

Platinum  thermometer,  71,  86, 
303 

Polarisation  of  light,  267,  272 

Polish,  104,  105 

Polonium,  200 

Porous  plug  experiment,  53, 

54 

Poynting,  J.  H.,  330,  334 
Pressure  of  radiation,  328  et 

seq. 

Prout,  254 
Psycho-physics,  14,  18,  37 


RADIATION,  176,  255,  270  et 

seq.)  282,  284  et  seg.,  296, 

328,  333 
Radio-activity,  3,  8,   186,   197 

et  seq. 
analysis   by   means    of, 

201 
Radio-activity,  decay  of,  212  ; 


curves,  fig.  32,  p.  215  ;  fig. 
33,  p.  223;  fig.  34,  p.  231 

Radio-activity,  energy  of,  198, 
223,  225,  236,  238 

induced  or  excited,  209, 

217,  232,  237 

of  ordinary  materials,  2 1 7, 

218,241,322 

of  the  earth  and  atmos- 
phere, 216,  219 

Radium,  200,  202,  207,  215, 
219,  221,  228,  230,  238,  240 

Ramsay,  Sir  Wm.,  62,  64,  65, 
233,  234,  319 

Rankine,  66 

Rayleigh,  Lord,  62,  63,  64 

Regenerative  process  of  lique- 
faction, 56 

Resonance,  296 

Reversal  of  spectral  lines,  297, 
307 

Reynolds,  Osborne,  289 

Richardson,  O.  W.,  187 

Roberts-Austen,    Sir    W.    C., 

95,  245 

Rontgen,  163,  164 
Rontgen  rays,   148,    163,    164 

et  seq. 

Roozeboom,  93,  94,  102 
Rotation  of  sun,  306 
Rowland,  169,  301 
Rutherford,  3,  203,  208,  211, 

222,  237,  239,  289,  322 

SALT  SOLUTIONS,  80,  82,  84, 

123,  128 

Saturation  current,  152 
Saturn's  rings,  312,  331 
Schulze,  138 


INDEX 


343 


Searle,  G.  F.  C.,  281 
Sea-water,  freezing  of,  84 
Secchi,  Father,  324 
Semi-permeable    membranes, 

no 

Sidgreaves,  Father,  326 
Silver,  86 

Soddy,  F.,  2 1 3, 2 19, 222, 233, 234 
Solar  radiation,  308 
Solid  solutions,  92,  96,  99,  101 
Solution,  problems  of,  8,  108, 

117 

Sorbite,  103 
Sorby,  104 

Spectro-helioscope,  317 
Spectroscope,  295  et  seq ,  313 

et  seq. 

Speculum  metal,  106 
Stars,  classification  of,  324 
Stars,  temporary,  325  et  seq. 
Stead,  T.  E.,  103,  104 
Steel,  101 
Steele,  B.  D.,  128 
Stellar  spectra,  324 
Stokes,   Sir  G.   G.,   159,   166, 

172,  296 

Stoney,  J.  3,  279 
Strutt,  Hon.  R.  J.,  204,  205 
Sugar  solutions,  no,  114,  123 
Sun-spots,  307,  318 
Sun's  age,  321 

-  energy,  320,  334 

temperature,  320,  334 

Surface  tension,  105,  107,  143, 

145,  156,248 


TELESCOPE,  294 
Temperature,  absolute,  45,  66, 
77 


Temperature  of  space,  334 

Thermodynamics,  i,  4,  26,  93, 
114,  119,  121,  122,  246 

Thomson,  J.  J.,  1,3,  148,  156, 
158,  159,  I73»  J78,  179,  180, 
181,  188,  203,  216,  217,  251, 
255,  257,  279,  281,  286,  287, 
288,  289,  319 

Thorium      and      Thorium-^, 

222,  224 

Thought-transference,  36 

Tin,  95 

Townsend,  J.  S.,  153,  179 

Traube,  109 

Tubes  offeree,  167  et  seq.,  193, 

270  et  seq. 
Tycho  Brahe,  325 


UNDULATORY    THEORY    OF 

LIGHT,  266,  270  et  seq. 
Units,  physical,  20,  38 
Uranium,  199,  203,  222,  240 


VACUUM  VESSELS,  fig.  i,  p.  61 
Valency,  138 

Van't  Hoff,  112,  113,  123,  146 
Velocity  of  stars,  305  et  seq., 

309  et  seq. 
Venus,    temperature    of    the 

planet,  335 

Viscosity  of  gases,  250,  264 
Vortex  rings,  263,  277 


WALD,  F.,  121 
Wallace,  Russell,  2^9 
Weight,  23,  27 
Weyprecht,  84 


344 


INDEX 


Whetham,  W.  C.  D.,  127,  129, 

139,  144 
Wilson,  C.  T.  R.,  156, 157,  158, 

I75»  J79>  216,  246 
Wilson,  H.  A.,  155,  175,  187, 

1 88,  236 

Wilson,  W.  E.,  321 
Wollaston,  295 


X  COMPOUNDS,  222,  224,  22$, 
229 

YOUNG,  266 

ZEEMAN,  284,  314 
Zeleny,  153 


THE   END 


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